CDS: Implement front-end support for local transactions

Implemented support on the TransactionProxy front-end for optimizations
where the Shard is local to the controller instance. In this mode, the
Shard's DataTree obtained from the FindPrimaryShard message is used to
prepare the modifications completely on the front-end. On ready, the
DataTreeModification instance is passed to the Shard via
the ReadyLocalTransaction message. The Shard then does a direct commit,
eliding the 3PC from the front-end (it's a no-op as it is for remote
write-only txns).

TransactionContext instances are now obtained via an
AbstractTransactionContextFactory passed to TransactionProxy of which
there are 2 kinds: one for single, unchained txns and one for chained
tnxs. Each creates a different DOM transaction instance to handle
preperation of modifications. The DOM transacton is wrapped in a
LocalTransactionContext which the TransactionProxy interfaces with.

Change-Id: I0322b586f394e4b6c7793b8287ac804b41964378
Signed-off-by: Robert Varga <rovarga@cisco.com>
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>