MRW37 - 4 readers + 2 writers

- Static allocation of dispatchers
- per bucket singleton notifier
- call getCouchBucket() only once inside create method

Change-Id: Id341ae350c69fc16a90908760f36b8b3521a964c
Merge branch 'kvshard6' of github.com:abhinavdangeti/ep-engine into kvshard6
MRW38: static allocation 4 readers + 2 writers

-flusher must continue checkpoint persistence for empty vbucket
-remove mutex in CouchNotifier::selectBucket()

Change-Id: I1541ea2fae2509b45684fc07e833cd8460f7e7e0