[SPARK-1112, 2156] use min akka frame size to decide how to send task results

Task results are sent either via akka directly or block manager indirectly, based on whether the size of the serialized task is smaller than spark.akka.frameSize. However, the result is actually sent back to the driver via the backend actor, which is initialized before receiving the SparkConf and hence it doesn't know the spark.akka.frameSize. That is the root cause of SPARK-1112.

A quick fix is using the min frame size to decide which route to go.

This PR also fixes SPARK-2156.
@kayousterhout I saw the line in `CoarseGrainedSchedulerBackend`. Should the overhead be bounded by a fixed size or proportional to the message size?