Only create a frozen Row on matching events

It is very wasteful to create a frozen row copy for every event
that we process. It can dramatically increase the time to process
the initial events from connecting to the database.

Closes-bug: #2011590
Change-Id: Ic4bf26d9b1f937073ddc6d0c3e9d22a777912ebf
(cherry picked from commit dea48cf)

Author: otherwiseguy

neutron/plugins/ml2/drivers/ovn/mech_driver/ovsdb/ovsdb_monitor.py

@@ -608,8 +608,9 @@ def __init__(self, driver):
             pass
 
     def notify(self, event, row, updates=None, global_=False):
-        row = idlutils.frozen_row(row)
         matching = self.matching_events(event, row, updates, global_)
+        if matching:
+            row = idlutils.frozen_row(row)
         for match in matching:
             self.notifications.put((match, event, row, updates))