Revert "Use spinlocks instead of conn mutex to guard UPR notifications"

This reverts commit 0d2696445ac6f584081938628f4bd936fe96a4d3.

Change-Id: I747d8dd59e9d1c365692961e8c9b1198149490d9
Reviewed-on: http://review.couchbase.org/37425
Reviewed-by: David Liao <david.liao@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>