Bug 7449: Add slicer Id to MessageSliceIdentifier

Both Shard and RaftActor (via AbstractLeader) (will) have separate
MessageSlicer instances and we need to determine to which instance
MessageSliceReply messages should be forwarded otherwise the first
MessageSlicer will drop messages destined for the second MessageSlicer.
Therefore add a slicerId field to MessageSliceIdentifier which is
checked by MessageSlicer#handleMessage.

Change-Id: Ib39ede29789d5bfaf1fdaea66a8d2994fe6ebcd6
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>