Add threading.RLock to guard ComputeAction._lock_registry from race conditions (OMS-559)
Fix issues discovered during limited testing (OMS-559)
Remove superfluous lock release call (oms-559)
Remove locking from a twisted callback (OMS-559)
Merge pull request #117 from opennode/oms-559

Resolve multithreaded race condition in ComputeAction resource locking (OMS-559)