Verify BatchedModifications messages sent vs received

A previous patch removed the verification of the response
Futures for BatchedModifications messages sent in
TransactionContextImpl. This was used to verify all modifications
were successfully received and processed by the transaction actor.

In lieu of that, this patch adds a totalMessagesSent field to
BatchedModifications and the transaction actor verifies it matches
the total number of messages received and successfully processed.

Change-Id: I247d3a09794142813cdca67b63d77412261a282b
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>