OVS agent: handle deleted ports on each rpc_loop iteration

OVS agent: handle deleted ports on each rpc_loop iteration

Currently rpc loop processes ports only in case polling is required
(message from ovsdb monitor) or there are port_updated notifications from
server or security group notifications.
In case of just port_deleted notifications port processing is not
triggered during rpc loop.
This may lead to agent accumulating a big amount of deleted ports
and processing all of them at once during next iteration when polling is
required or any notification from server, which might be quite tough for
the agent. Tough means agent will be irresponsive while processing deleted
ports.
The patch makes port deletion processing more gradual.

Closes-Bug: #1491922
Change-Id: I0e1f6dfbf5b56fb18a978d6214e1768560d8ac98