Add synchonization thread in update_port

Creating/updating VIP in LbaaS quickly ends up with a LockTimeout in the
pool object. Adding the utils.synchronized() decorator, it avoids this
problem.

Fixed-bug: https://midobugs.atlassian.net/browse/MN-1459
Merge pull request #98 from midokura/bug/mn-1459

LbaaS LockTimeout error solved