*Motivation*

Currently KeyValue schema doesn't support using AUTO_CONSUME.

This PR is to add this support.

This PR is based on #4836

*Changes*

- refactor a bit on Schema interface to support fetching schema info for both AutoConsumeSchema and KeyValueSchema before subscribing
- add AUTO_CONSUME support to KeyValueSchema
- add tests

* Add maven plugin for style checking. Fix some style violations.

* Fix issues shown by the style checker in the pulsar-flink module

*Motivation*

Currently fetching schema information is done synchronously.
It is called in netty callback threads and will potentially block
async operations.

*Modifications*

Make most of the operations asynchronously in SchemaInfoProvider.

### Motivation
The original PR https://github.com/apache/pulsar/pull/4770 is merged before I could give some feedback, so I just create a new pull request.

### Modifications
1. Refine the structure, remove some unnecessary info currently. Thus it is displayed to users as a complete guide. 
2. Refine the language part, make it clear and concise.
3. Though "upgrade ZooKeeper" is optional, could we add information for it? So users can decide whether they want to upgrade ZooKeeper. When they upgrade ZooKeeper, they know how to upgrade.

fix #4755 
Retention policy is used in Ledger GC and Topic GC.
Default retention policy is not uploaded to zookeeper, but it is getted from zookeeper when it is used.
So, if zookeeper doesn't have retention policy, we should load it from default config file.
In Ledger GC configuration, it's OK.
https://github.com/apache/pulsar/blob/075f28b71c8fd9259ce8e136dbb81c0629c3f271/pulsar-broker/src/main/java/org/apache/pulsar/broker/service/BrokerService.java#L739-L742
But in Topic GC, do nothing.

s/is/it/

* Add develop part

* Add new site2/docs/functions-develop.md for new structure

* Adopt advice from Jerry

* Update Python and Go examples

* remove some tabs for Go

* add Go example in logger section

* remove Encryptiion at the end

* restructure Example in SerDe section

### Modifications

When submitting a function via REST interface, if user doesn't provide the function package or function runtime as part of function config, the function will get submitted and deployed to run but the typeClassName for the source will not be set causing a weird error in the function instance code

### Motivation

A message doesn't get automatically ACKed if there is the output topic is not set

* Simplified assert statements in the tests. Switch to usage of static imports in tests. (Part 1)

* Simplify assert statements in the tests and use the appropriate assert statements. Switch to usage of static imports in tests. Remove unused imports (Part 2)

* Process requests asynchronously on some REST APIs (3)

* Add async rest handler to API for expiring message on single topic subscription

*Motivation*

Add a few recent presentations to the resources page. They cover different topics:

- 2.4.0 release
- use case
- serverless
- spark + pulsar
- flink + pulsar

Add an independent Chapter for Pulsar Schema.

This is the first section—Get started.

### Motivation
Continue the PR of #4151

fix #4707 

* [Transaction][Buffer]Add new marker to show which message belongs to transaction
---

*Motivation*

Add new message type in the transaction including data and commit and abort maker in the transaction log.

*Modifications*

Add two new types of transaction messages.
TXN_COMMIT is the commit marker of the transaction.
TXN_ABORT is the abort marker of the transaction.

Master Issue: #4756

### Motivation

This is a continuation of https://github.com/apache/pulsar/pull/4765.

### Modifications

Added async rest handlers to the following APIs:
```
DELETE /admin/namespaces/{tenant}/{cluster}/{namespace}
PUT    /admin/namespaces/{tenant}/{cluster}/{namespace}/unload
POST   /admin/namespaces/{tenant}/{cluster}/{namespace}/clearBacklog
POST   /admin/namespaces/{tenant}/{cluster}/{namespace}/clearBacklog/{subscription}
POST   /admin/namespaces/{tenant}/{cluster}/{namespace}/unsubscribe/{subscription}

DELETE /admin/v2/namespaces/{tenant}/{namespace}
PUT    /admin/v2/namespaces/{tenant}/{namespace}/unload
POST   /admin/v2/namespaces/{tenant}/{namespace}/clearBacklog
POST   /admin/v2/namespaces/{tenant}/{namespace}/clearBacklog/{subscription}
POST   /admin/v2/namespaces/{tenant}/{namespace}/unsubscribe/{subscription}
```

* Added more blog posts in the resources page

* Added 2 more posts

### Motivation

Fix #4732 

### Modifications

Add options to rewrite the namespace delimiter, disable by default

Enable rewrite namespace delimiter can work well with superset:
<img width="1279" alt="superset" src="https://user-images.githubusercontent.com/12592133/61385412-f0f35700-a8e4-11e9-87b2-a31b62128b58.png">


### Does this pull request potentially affect one of the following parts:

*If `yes` was chosen, please highlight the changes*

  - Dependencies (does it add or upgrade a dependency): (no)
  - The public API: (no)
  - The schema: (no)
  - The default values of configurations: (no)
  - The wire protocol: (no)
  - The rest endpoints: (no)
  - The admin cli options: (no)
  - Anything that affects deployment: (no)

### Documentation

  - Does this pull request introduce a new feature? (no)

* Add Upgrade Guide to Apache Pulsar

*Changes*

Add a general upgrade guide to apache pulsar.

* Update the upgrade guide

* [Pulsar-Client] Add Producer Numeric Properties Validation

* Aligned deprecated and new Producer API validations

* Deprecated and new Producer API validations are being aligned

* batchingMaxMessages C++ API is being aligned with Java API

* batchingMaxMessages Java API Validation is being aligned with C++ API

* Review comments are addressed

* Fix broken UTs

* Convert anonymous functions to lambda

* Replacing lambda with anonymous implementation, because lambda cannot be mocked

### Motivation

`pulsar-function-go/conf` package apply `instance-conf-path` with default value `HOME_PATH+github.com/apache/pulsar/pulsar-function-go/conf/conf.yaml`, once function deployed, the running node may not have the yaml conf file exist, then go function will panic with `not found conf file` error. 

This PR changed the logic of config parsing, parse `confContent` first, then parse `confFilePath` if `confContent` empty. 

### Motivation

After the changes in #3118, there has a been a sharp increase of memory utilization for the UnackedMessageTracker due to the time buckets being created. 

This is especially true when the acktimeout is set to a larger value (eg: 1h) where 3600 time-buckets are being created. This lead to use 20MB per partition even when no message is tracked.

Allowing to configure the tick time so that application can tune it based on needs.

Additionally, fixed the logic that keeps creating hash maps and throwing them away at each tick time iteration, since that creates a lot of garbage and doesn't take care of the fact that the hash maps are expanding based on the required capacity (so next time they are already of the "right" size). 

On a final note: the current default of 1sec seems very wasteful. Something like 10s should be more appropriate as default.

*Motivation*

When using PulsarService or BrokerService for testing, it might require accessing
the components in PulsarService and BrokerService. This change is adding setters
and getters to access the components in PulsarService & BrokerService

### Motivation


Currently, if the kubernetes namespace set to deploy functions in is different than the one in which brokers/workers reside, get status and stats doesn't work because the url for instances does not specify the namespace.

### Motivation

Switched back to use the regular `java.util.concurrent.CopyOnWriteArrayList` instead of the class extending it since we don't really have any advantage in accessing the underlying array of objects. 

The reflection being used to get that field is giving errors on Java 12. 

### Motivation

There are some typos in the document content of about debezium, which affect users' use, so fix it.

### Modifications

Fix typos in document of debezium. And format content 

* Changed remove-backlog-quotas to remove-backlog-quota

* Changed remove-backlog-quotas to remove-backlog-quota

* Increased the consumerName field to varchar(256)
Signed-off-by: NYuvaraj Loganathan <uvaraj6@gmail.com>

* add memory requirement and config tips for standalone mode

In the current standalone get start doc, there is no mentioning or link for the memory usage of the pulsar. 
User with limited free memory may encounter issue to start the Pulsar. 
Adding the tips on how to change the default required heap memory.

* [docs] add security warning on standalone doc

Add a fair warning on standalone get start document.  Since by default configure, Pulsar can be accessed from remote server without any authentication, encryption, authentication.  So a fair warning to the user is critical to avoid any unexpected security risks.

### Motivation

Currently golang function needs to be compiled before deploy to pulsar, so the executable permission is required when function package is downloaded to local node from bookkeeper. This PR is intent to make golang function package executable after download from bookkeeper, to make sure the function is ok to run.

**Motivation**
when create partitioned topic, there is a check that `numPartitions > 1`, if numPartitions==1, it will fail.
Some user may want to create partitioned topic with only 1 topic at the start time, and during using it, could update to more topics later. 

**Modification**
change check of `numPartitions > 1` to `numPartitions > 0`

expect all existing ut passed.