Merge branch 'stable-3.6' into stable-3.7

* stable-3.6:
  Set project and Gerrit version to v3.5.6
  Remove Maven legacy pom.xml
  Make events-broker a Gerrit plugin
  Provide subscribers with consumer's group id
  Remove actual implementation of InProcessBrokerApi
  Set event-broker and gerrit version to v3.4.8
  Add interface ExtendedBrokerApi to consume messages with groupId
  Change the build result name to events-broker.jar
  Set version to 3.6.3

Change-Id: I559ee13c4c2b57aded856e6763b1e68557c8ef43