BUG-7033: Allow ShardDataTree to pipeline transactions

BUG-7033: Allow ShardDataTree to pipeline transactions

InMemoryDataTree gives us the DataTreeTip, which allows another
DataTreeModificatoin to be prepared on top an uncommitted one. This
allows us to pipeline transactions if we manage our lifecycle properly.

For now this feature gives stricter abort validation, and has otherwise
no impact. In future, this allows another DataTreeModification to be
prepared and queue for persistence before replication of the previous
one finishes.

Change-Id: I7ab97c906a6403da780800edd335f74c403e5aa4
Signed-off-by: Robert Varga <rovarga@cisco.com>
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>