incorrect errors from concurrent transactions

[Mike Thompson]

The AppDB/appscale_server.py code expects the zookeeper acquireLock to raise an exception when a given lock is already in use. However the acquireLock code returns "False" in such a case and does not raise an exception. Thus, the application does not see the "CONCURRENT_TRANSACTION" exception, it ends up seeing some side effect of attempts to delete an unknown transaction because the PB Server logic proceeds as if the lock were acquired.