More efficient pruning of events
More efficient pruning of events

Avoid a potentially large select in clause. synchronize_session may be
False here, as we don't have to worry about old stale pruned events in
the session (keeping in mind the prune gets triggered when new events
are created).

Change-Id: I27a00a1afb6518176c031551d6a74e8b12546ef2

noironetworks/heat,noironetworks/heat,openstack/heat,openstack/heat