Slice front-end request messages

Slice front-end request messages

Added infrastructure to use the MessageSlicer to slice SliceableMessages in
the TransmitQueue.Transmitting class on the front-end. A MessageAssembler is
used on the Shard side to re-assemble. Currently only the
front-end ModifyTransactionRequest is a SliceableMessage as it contains a
NormalizedNode which can be arbitrarily large - the others are small and
don't require slicing.

Change-Id: I7b09e4864e19d3fdb215c2b9dbcb64c14b6a143c
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>