Added explicit locking of the data change listener / commit handler registration

  - This locking fixes ConcurrentModificationException in cases
    when transaction processing is computing affected commit handlers
    and data change listeners.

Change-Id: Icdd9a2ea36102f681a5d82390c3b9d942560b878
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>
Merge "Added explicit locking of the data change listener / commit handler registration"