Bug 2160: Added concurrent 3-phase commit coordinator

Added a new class DOMConcurrentDataCommitCoordinator which is an
implementation of DOMDataCommitExecutor that allows multiple concurrent
3-phase transaction commits. The 3 phases are still performed serially and
non-blocking per transaction.

A new flag, "allow-concurrent-commits", was added to the DOM in-memory
data broker's yang config. If true, the new
DOMConcurrentDataCommitCoordinator implementation is used, otherwise the
current implementation is used. The default is false.

The distributed data store will use the concurrent coordinator.

Change-Id: Ie21947d6f9c1086f89686a73442c854c08c07100
Signed-off-by: tpantelis <tpanteli@brocade.com>