BUG-509: make sure datastore commits are visible

BUG-509: make sure datastore commits are visible

This wraps the datastore root in an AtomicReference, which makes sure
started transactions see the latest published commit. This will make
the datastore handling more robust under tight conditions.

Also uncovers the fact that we are invoking user code under lock, which
we fix by reusing the executor used by the commit machinery.

Finally it uncovers thread-unsafe listener list manipuation. This will
need to be addressed in a follow-up patch.

Change-Id: Ic7efd266ef680701c1f0944ee675122d8527568b
Signed-off-by: Robert Varga <rovarga@cisco.com>