Rework CDS commit cohort impl to handle yang lists

Rework CDS commit cohort impl to handle yang lists

If a cohort registers for yang list entries, it works fine if the
transaction only contains a write or delete of one list entry.
However the DataTreeCohortActor throws an UnsupportedOperationException
from CohortBehaviour#handle if more then one list entry is written. In
that case multiple CanCommit messages are sent to the DataTreeCohortActor,
a DOMDataTreeCandidate for each entry, but the CohortBehaviour is set up
to only handle one message, after which it expects to transition to the
PostCanCommit step.

It seems the DOMDataTreeCommitCohort#canCommit API really should take
a collection of DOMDataTreeCandidates. Howeever in lieu of an API change,
I modified the CanCommit message to contain a collection of
DOMDataTreeCandidates. The DataTreeCohortActor invokes canCommit for
each one and uses the last PostCanCommitStep returned. This *may*
be OK although there doesn't seem to be an alternative at this point.
We probably should note this behavior in the DOMDataTreeCommitCohort
API.

Change-Id: I17c4d2f477ffc6c6c3921217e5f6c13bcdafde8f
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>