Bug 2264: Use streaming for snapshots

Bug 2264: Use streaming for snapshots

The NormalizedNode snapshot payload is now streamed.

On the Shard side, added a new message CreateSnapsot to the
ShardReadTransaction to read the data tree root, serialize it, and
return a CaptureSnapshotReply message. On createSnapshot, the Shard
now sends the CreateSnapsot message to the read-only tx actor instead
of a ReadData message. This moves the serialization out of the Shard.

On the RaftActor side, the Snapshot class remained the same as it stores
a byte[] and is already Serializable. The CaptureSnapshotReply now
stores a byte[] instead of ByteString. The internal RaftActor code for
capture and apply snapshot was changed to use/pass byte[] to eliminate
the overhead of converting to/from ByteString.

Change-Id: Id12677441dce54bebbb5b71c68cf457d7c915ba1
Signed-off-by: tpantelis <tpanteli@brocade.com>