Merge branch 'stable-3.6' into stable-3.7

* stable-3.6:
  Do not rely on async/wait for synchronous fetch replication
  Cover the replication failure scenario and fix the metrics
  Remove white-box unit tests on synchronous FetchCommand
  Throw Exception from tests

Change-Id: I91fd3967d0c17eb5645b0531bb6986478ad3b8e2