Fix possible infinite recursion in MessageTransferModule.SendGridInstantMessageViaXMLRPCAsync() whilst preserving retry lookup behaviour.

This is based on heavily mikemig's original patch in http://opensimulator.org/mantis/view.php?id=7149
but instead of exiting after the first IM delivery failure to presence information retrieved from the presence service
it will retry the lookup until the result matches the previous lookup.
This is to deal with the case where the agent is sent an IM whilst they are teleporting.