MB-7966 singleton CouchNotifier

This implements singletone CouchNotifier class that is an alternative
approach for synchronization among concurrent notification requests
from mutiple dispatchers.

Change-Id: I13902e9b6161dd177c3e643118b57098ad3cf056
Reviewed-on: http://review.couchbase.org/26034
Tested-by: Jin Lim <jin@couchbase.com>
Tested-by: abhinav dangeti <abhinav.dangeti@gmail.com>
Reviewed-by: abhinav dangeti <abhinav.dangeti@gmail.com>