Fix Go routine race condition in DB notification map

Fix Go routine race condition in DB notification map

Map cbMap was accessible by the main Go routine and also by a Go routine, which
handled incoming notifications from database (Redis). Problem was visible, when
unit tests were run with '-race' flag. Fix the issue by adding a mutex lock to
guard cbMap accesses. Go routine what handles incoming notification has a local
lockless copy of the cbMap to make sure that notification handling speed won't
be suffering from mutex locks. Local copy of cbMap is updated whenever the
cbMap has been updated.

Signed-off-by: Timo Tietavainen <timo.tietavainen@nokia.com>
Change-Id: I066ff1d71340db2240a7ea6aeeb575f098488608