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: I86ac07d29f3575d334f3b7f27530dc6019adf294
Reviewed-on: http://review.couchbase.org/36909
Tested-by: David Liao <david.liao@couchbase.com>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>