Use spinlocks instead of conn mutex to guard UPR notifications
similar to the way TAP does. This will decrease lock contention
since conn mutex is no longer needed here.

We now need to manipulate vbconns structure for UPR too:
add and remove connection for a stream based on vbid.

Also Refactored ConnMap/TapConnMap/UprConnMap as needed.

Change-Id: I86ac07d29f3575d334f3b7f27530dc6019adf293
Reviewed-on: http://review.couchbase.org/35050
Tested-by: David Liao <david.liao@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>