StoreGate: Fix potential deadlock between SGImplSvc and DataProxy

The SGImplSvc::proxy methods take out a lock on the store, fetch
the proxy, then call isValid() on the proxy.  DataProxy::isValid()
will acquire a lock on the proxy and than can call back to the store
via TransientAddress::contextFromStore().  Thus, we can get a deadlock
if SGImplSvc::proxy() is called in one thread and DataProxy::isValid()
in another.

Resolve by releasing the SGImplSvc lock in proxy() before calling
DataProxy::isValid().