1. Add new setting for Kafka engine, named kafka_thread_per_consumer which default value is false. 2. Create separate thread pool for Kafka engine.

Follow-up-for: #10315
Follow-up-for: #10263

* Move some code outside dbms/src folder
* Fix paths

Signed-off-by: NSlach <bloodjazman@gmail.com>

add system.metrics.MySQLConnection
Signed-off-by: NSlach <bloodjazman@gmail.com>

This reverts commit 2ddb801d.

Add VERSION_INT (ClickHouse version in integer format like 18014009) to system.metrics and system.build_options

- We add BackgroundSchedulePool which can execute a function at a specific point in time. Basically all tasks are added in a queue and precessed by worker threads.
- The most important difference between this and BackgroundProcessingPool is that we have the guarantee that the same function is not executed from many workers in the same time
- Each of the following classes instead starting a thread will register a task in BackgroundSchedulePool and when they need to run will call schedule or scheduleAfter(duration) functions

This commit is moving all threads created by ReplicatedMergeTree to BackgroundSchedulePool tasks

NOTE: I did a minimum number of changes to be much simple to review the code

This reverts commit abaa1501.

* Profiling external cached dictonaries [#CLICKHOUSE-2197]

* style