OFAgent: Process port_update notifications in the main agent loop

Port the following patch to OFAgent.
commit: 5e6e592132aa9a98936ce3bfdb66efc7832caafb
https://review.openstack.org/#/c/61964/

Partial-Bug: 1293265

Change-Id: I53813d12c66dc746cd373fd91ff9bd9bdbf222db
OFAgent: Improve handling of security group updates

Port the following patch to OFAgent.
commit: 3046c4ae22b10f9e4fa83a47bfe089554d4a4681
https://review.openstack.org/#/c/63100/

Partial-Bug: 1293265

Change-Id: Iecdfee894ff2fd5f05f63f040dd70821af124737