Fix #1435

* #1447

Fixed the after method interceptor for the apache http client NPE error when the response is null cause by the doExecute method throw a exception.

Make the icon font things more clear in NOTICE, include where they are in our source folder. Also move the text at the beginning of the distribution NOTICE file. (#1444)

* Cluster module implementation finished.

* Fix #1441 add icon files notice

* Amend notice file, add excluding icon folder in rat config

Sync English documents to Chinese.

* Add Junit test for ribbon's NIWSServerListClassName

* Add License Head to Code file

* try to solve submodule skywalking-ui conflict

# Conflicts:
#	README_ZH.md

Init branch 6.0 for our next big iteration. Change root readme. Format field document in table.  Remove Chinese documents.

Fixed the bug of the result of GC count in query of GC trend service always 0.

* Update How-to-release.md

* Update create_source_release.sh

* Update README.md

* Update README_ZH.md

Error rate alarm is not triggered

* Update submodule skywalking-ui

* Update submodule skywalking-ui

* Update submodule skywalking-ui

* Update submodule skywalking-ui

* #1405
Add application id into service metric for the page jump from overview page to service page.

* Fixed test case error.

* Update submodule skywalking-ui

* Update submodule skywalking-ui

* Update submodule skywalking-ui

* Fixed the wrong usage of elasticsearch filter.

* #1393

The array data type of service name column are nested. Change the data type to be just a one-dimensional array.

* Update submodule skywalking-ui

* Update submodule skywalking-ui

* Prepare beta2 changelogs.