[SPARK-1112, 2156] (1.0 edition) Use correct akka frame size and overhead amounts.

SPARK-1112: This is a more conservative version of #1132 that doesn't change
around the actor system initialization on the executor. Instead we just
directly read the current frame size limit from the ActorSystem.

SPARK-2156: This uses the same fixe as in #1132.
So that change actually alters the expectations of the unit tests, so I went ahead and just enforced the limit in the LocalBackend anwyays.