gom/resource: avoid weak pointer usage

Weak pointers are extremely slow in GObject and we don't really need to use that anyway. We really don't have a good multi-threaded story here, but I'm using a GMutex around the repository just to try to keep things up with whatever they were before naturally in add_weak_pointer(). There is a lot of unsafe things that can happen once you start threading anyway.

This speeds up resource finalization considerably.