Avoid dhcp agent race condition on subnet and network delete

Avoid dhcp agent race condition on subnet and network delete

Ensure that ports that are about to be deleted are 'selected
for update'. By doing so, we avoid a race condition between
subnet and network delete operations carried out by two
separate server instances.

A race caused by the dhcp agent deleting the DHCP port
(caused by a subnet-delete event notification) can
still occur and will be addressed in a subsequent patch.

delete_subnet's way to delete ports has been tweaked to
ensure that postgres db can handle the SELECT FOR UPDATE
correctly.

Partial-Bug:1197627

Change-Id: I5bd75a758395a2faeff9db35a03c42dfa8ae0eab