GCancellable: Ensure it is always cancelled on connect callback

When a cancellable is cancelled when we call g_cancellable_connect we
used to immediately call the provided callback, while this is fine we
actually had race in case the cancellable was about to be reset or in
the middle of a cancellation.

In fact it could happen that when we released the mutex, another thread
could reset the cancellable just before the callback is actually called
and so leading to call it with g_cancellable_cancelled() == FALSE.

So to handle this, make disconnect and reset function to wait for
connection emission to finish, not to break their assumptions.

This can be tested using some "brute-force" tests where multiple threads
are racing to connect and disconnect while others are cancelling and
resetting a cancellable, ensuring that all works as we expect.
9 jobs for fix-cancellable-connect-race in 24 minutes and 41 seconds (queued for 1 minute and 55 seconds)