Merge branch 'stable-2.16'

* stable-2.16:
  Replace DestinationFactory with Guice generated Factory and assisted injection
  Revert "DestinationFactory: remove @Singleton annotation"
  DestinationFactory: remove @Singleton annotation
  Updated docs to clarify that SSH config is not optional
  Introduce DynamicSet of ReplicationStateListener
  Remove unneeded RemoteSiteUser.Factory
  Make SecureCredentialsProvider public for reuse

Change-Id: I52bc2580bef89d40f9b02749550cdbbd6e7a26dc