thread-mainloop: Add API for running a callback unlocked

This adds API to allow clients to schedule a callback in the mainloop thread without the mainloop lock being held. This is meant for a case where the client might be dealing with locking its own objects in addition to the mainloop thread itself. In this case, it might need ton control the locking order of the two, to match the order in other threads, as it might not always be able to allow for its objects to be locked after the mainloop thread lock.