Merge branch 'stable-3.8'

* stable-3.8:
  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
  Bump version to 3.7.2
  Set version to 3.6.3

Change-Id: I2d7c9fa0cb21031681bc539348923c31b77d7889