Update drain_queues to check if queue has already been drained

When GeventScheduler is being used as async_.schedule there is a chance
the queues are being drained while a drain_queue job is scheduled to be
executed on the gevent event loop. This is valid so change assert is_tick_used
to check if queue has already been drained instead of throwing an assertion
exception.

promise/async_.py

@@ -108,11 +108,11 @@ def wait(self, promise, timeout=None):
         self.schedule.wait(target, timeout)
 
     def drain_queues(self):
-        assert self.is_tick_used
-        self.drain_queue(self.normal_queue)
-        self.reset()
-        self.have_drained_queues = True
-        self.drain_queue(self.late_queue)
+        if not self.is_tick_used:
+            self.drain_queue(self.normal_queue)
+            self.reset()
+            self.have_drained_queues = True
+            self.drain_queue(self.late_queue)
 
     def queue_tick(self):
         if not self.is_tick_used: