ofagent: Stop monitoring ovsdb for port changes

Perform dumb polling instead.
Now ofagent port monitoring is mostly ovsdb-free.
(except _find_lost_vlan_port stuff, which is planned to be retired
by blueprint ofagent-merge-bridges)

Partially-Implements: blueprint ofagent-port-monitor
Change-Id: Ib4701f7c8ea0ee03229d207c0fbbf6f42a55aecb
Merge "ofagent: Stop monitoring ovsdb for port changes"