central.update_status should be transactional

Central's update status method issues a large number of SQL
queries for large zones with many recent updates, as a result
it's checking out many connections from our connection pool one
after another, contributing to bug 1445123. By wrapping the
call in a transaction, a single connection will be checked out
of the pool.

Partial-Bug: 1445123
Change-Id: Idfde7578e1a17a6a0585835fe614f787102563fa

designate/central/service.py

@@ -2135,6 +2135,7 @@ class Service(service.RPCService, service.Service):
         return pool
 
     # Pool Manager Integration
+    @transaction
     def update_status(self, context, domain_id, status, serial):
         """
         :param context: Security context information.