Context

AsyncMongoClient.cleanup_cursor is a coroutine, making it uncallable from any __die_ methods. This prevents asynchronous cursor classes from correctly cleaning up at GC time. We need to separate out the code path in AsyncMongoClient._cleanup_cursor that allows locks from the lock-free code path in order for async cursors to cleanup. 

Definition of done

AsyncMongoClient._cleanup_cursor must be synchronous and not attempt to lock.

Pitfalls

Everywhere we use AsyncMongoClient._cleanup_cursor will have to explicitly call the locking or lockless version after this change.