First successful exchange with NM ShuffleHandler

This fixes two major things:
(1) There is a race condition in the Yarn auxiliary service filed
    under MAPREDUCE-6116. We need to manually synchronize around
    starting containers if we want to set shuffle service data.
(2) The message from the AM to the driver was always dropped.
    While the exact cause for this drop is unknown, the minimal
    it needs to send is a simple Array[Byte] that represents the
    serialized job token. Then the driver can later reconstruct
    the key from these bytes.