* Support direct server setting in agent.

* Fix format.

* Add direct_servers check (#981)

Fixing javadoc. And try to trigger a ci fail in this commit, in order to make sure javadoc will be checked in further pull request.

Fix a typo.

1. Remove the “apm-trace” module, isolate the trace structure between agent and collector for considering performance.
2. Change the head name.
3. Provide potienal mechanism for ignore trace with certain suffix.

add test case for okhttp interceptor

* Add mechanism to disable any or all plugins, Relate: #186

Fix typo, and remove unnecessary lock in NoConcurrencyAceessObject. Refact FileWriter and SnifferConfigInitializer.

Finish CollectorClient code. It is almost ready to send TraceSegement (s) to collector in batch/fail-switch mode.

Fix compile issue. Add new rest-service Object: SegmentsMessage.java. It uses Gson(JsonAdapter) as serialize/deserialize tool.