GCancellable: Use per-instance mutex logic instead of global critical sections
GCancellable is meant to be used in multi-thread operations but all the cancellable instances were sharing a single mutex to synchronize them which can be less optimal when many instances are in place. Especially when we're doing a lock/unlock dances that may leave another thread to take the control of a critical section in an unexpected way.
This in fact was leading to some races in GCancellableSources causing leaks because we were assuming that the "cancelled" callback was always called before our dispose implementation.
The lock is also now used only to protect the calls that may interact with cancelled state or that depends on that, as per this we can just reduce it to the cancel and reset case, other than to the connect one to prevent the race that we could have when connecting to a cancellable that is reset from another thread.
We don't really need to release the locks during callbacks now as they are per instance, and there's really no function that we allowed to call during a ::cancelled signal callback that may require an unlocked state. This could been done in case with a recursive lock, that is easy enough to implement but not really needed for this case.
Coming from !2759 (merged)