BigSwitch: Sync workaround for port del deadlock

Adds a synchronization barrier to the methods
that lock the port table to avoid the mysql
update-lock/eventlet-yield deadlock that occurs
with simultaneous port deletions.

Closes-Bug: #1292232
Change-Id: I8975a14a937a5843a0395ad7b599ebdc827565ed
Merge "BigSwitch: Sync workaround for port del deadlock"