BUG 2792 : Serialize phase processing in ConcurrentDOMDatBroker

BUG 2792 : Serialize phase processing in ConcurrentDOMDatBroker

Ensure that any given submit phase is completed for a cohort before
proceeding to run the same submit phase on the next cohort in the
transaction.

For example if a transaction has 2 cohorts then canCommit on cohort1
must complete before we proceed to canCommit on cohort2.

If we try to concurrently try to process the phase on all cohorts in
a transaction it can lead to deadlocks as outlined in the bug

Change-Id: I4e758770c711d92920a456e2ac3757ebbb63eb46
Signed-off-by: Moiz Raja <moraja@cisco.com>