Protect contentious transactions with a semaphore

    The create_port and delete_port request generates
    highly contentious transactions to (de)allocate
    an unique IP addresses updating a single table.
    Exclusive Locks work better than transactions to
    handle concurrency here.
Merge pull request #26 from midokura/takayuki/protect-transaction-with-semaphore

Protect contentious transaction with a semaphore