LbaaS LockTimeout error solved

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

    Once it will be merged, I will perform a cherry-pick over icehouse branch and ask for another pull request.

    Fixes bug: https://midobugs.atlassian.net/browse/MN-1459