Fix unregister service bug in GravityNode

[vdipernaAPS]

If a registration timestamp is not set in a ServiceDirectory unregister request from a GravityNode, the unregister can fail.

[cbrundick]

The previously implemented fix introduces the potential for a lock leak. If (urlInstanceMap.count(url) == 0), the call to unregisterService returns without releasing the serviceManagerSWL.lock