IOVSvc: Try to fix potential deadlock between StoreGate and IOVSvc.

IOVSvc::createCondObj locks IOVSvc and calls SGImplSvc::proxy, which locks the store.

SGImplSvc::regFcn and friends lock the store and then call IOVSvc, which tries to acquire its own lock.

Thus we have a potential deadlock.

Change IOVSvc::createCondObj so that it's not holding the IOVSvc lock when it calls proxy().