Fix race condition on ml2 delete and update port methods

Synchronize access to ports table when deleting and updating
a port. Otherwise concurrent update/delete request for the same port
may cause neutron server to throw an exception and return
'500 Internal server error' for such requests.

Change-Id: I868002643147ce6baace5671cffb38b4f5e66729
Closes-Bug: #1266537
Merge "Fix race condition on ml2 delete and update port methods"