Back off if another thread has processed all messages for this actor. See #3064

Conflicts:
	akka-testkit/src/main/scala/akka/testkit/CallingThreadDispatcher.scala
Merge pull request #1299 from akka/wip-3180-backport-callingthreaddispatcher-fixes-ban

2.1 Backport of CallingThreadDispatcher fixes to prevent test deadlocks.