* Remove javassist dependency

*   Finish the PR.

* Change doc title level.

* Remove the duplicated codes.

* POC: Intercept successfully.

* Refactor codes

* Build the new core

* Setup the new core.

* Push the javassist version to 3.23.2-GA, as it is the last version supports JDK 1.6 compile, according to https://github.com/jboss-javassist/javassist/blob/rel_3_23_2_ga/pom.xml#L149-L160

* Finish the all codes.

* Document updated for bootstrap instrumentation.

* Add a notice to document.

* Remove unexpected file.

* Set javassist version right. Add judge in core interceptors

* Fixe no log bridge implementation

* [maven-release-plugin] prepare release v6.2.0

* [maven-release-plugin] prepare for next development iteration

* fix apm-agent-core maven pom file shade-plugin define bug (#2769)

* Move submodule plugin.

* Always auto-submodule

* [maven-release-plugin] prepare release v6.1.0

* [maven-release-plugin] prepare for next development iteration

* Change keys location.

1. Fixed the warning of 'parent.relativePath' points issue when running the mvn package command. (#2404)

2. Fixed the warning of duplicate declaration of plugin issue when running the mvn package command.
3. Fixed the warning of duplicate declaration of mysql connector jar when running the mvn package command, because of provided scope include test scope.
4. Fixed the warning of expression ${artifactId} is deprecated issue when running the mvn package command.
5. Fixed the warning of expression ${version} is deprecated issue when running the mvn package command.

* [maven-release-plugin] prepare release v6.0.0-GA

* [maven-release-plugin] prepare for next development iteration

* feat:Active System.env Config In Both Collector And agent.config.
1、moving related codes into apm-util module.
2、update Setting-override.md about how to use.

* resolve docs.

* typo

* finish test case.

* Update README.md

* Lock oracle and resin plugins version.

* Move resin and oracle plugin to openskywalking

* Restore submodule active.

* Add another submodule active command.

* Remove the wrong test cases in release and fix some doc issues.

* [maven-release-plugin] prepare release v6.0.0-alpha

* [maven-release-plugin] prepare for next development iteration

* Fix some doc

* Fix mesh receiver doesn't update heartbeat.

* Fix bug of mesh heartbeat and rest receiver.

* Fix issue: opencensus is included in agent without shade. @tuohai666

* update 5.x grpc version form 1.10 to 1.14 and  Solve the resulting guava and log4j version conflicts

* Specify guava version 20.0

* update parrent pom guava version

* Moves to Maven wrapper to stabilize the version in use

This will ensure those building will have similar experience due to
having a fixed maven version.

* Updates most plugin versions and removes redundant configuration

* Sets up build enforcement for JDK 1.8 until further work occurs

Remove collector.servers and collector.direct_servers, and add collector.backend_service setting. cc @ascrutae

* Change version 

* Finish the prototype of TLS of gRPC between agent and server.

* Fix for rat check.

* Update GRPCChannelManager.java

* Update AgentModuleGRPCProvider.java

* Upgrade netty version (#971)

* Make sure end user can't start gRPC server with and without SSL in the same port.

* [maven-release-plugin] prepare release apm-5.0.0-alpha

* [maven-release-plugin] prepare for next development iteration

Fix assmbly for mvn clean install -Papache-release and update version to SNAPSHOT for SNAPSHOT release.