[merge commits require]Sync 6.0 to master

* Buffer stream.

* Buffer file reader.

* Buffer library performance test and functional test successfully.

* Fixed the code merge mistake.

* SkyWalking's trace receiver implementation.

* SkyWalking's trace receiver implementation.

* Delete collector module.

Add a lot of documents. @peng-yongsheng @hanahmily . Also @hanahmily , the k8 cluster management document and deploy document need you to add.

# Conflicts:
#	docs/powered-by.md

* add user

* add user

* FIx NPE in InstanceMetricAlarmAssertWorker

alphabeta to alphabet

* Buffer stream.

* Buffer file reader.

* Buffer library performance test and functional test successfully.

* Fixed the code merge mistake.

* fix memory leak in HystrixPlugins

* add heart beat handler to jetty

* add heart beat handler to jetty

* 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

# Conflicts:
#	docs/powered-by.md

* 用户接入 上上签

* add user

* Buffer stream.

* Buffer file reader.