Reset state to READY before completing futures

This should fix a race condition where we:

1. say we're ready for new work by completing the predict future
2. this updates healthcheck status by setting runner back to "not busy"
3. we haven't updated worker state to READY yet, so the next prediction
   fails

python/cog/server/worker.py

@@ -185,9 +185,13 @@ def _consume_events_inner(self) -> None:
             return
 
         assert self._result
-        self._result.set_result(done)
+
+        # We capture the setup future and then set state to READY before
+        # completing it, so that we can immediately accept work.
+        result = self._result
         self._result = None
         self._state = WorkerState.READY
+        result.set_result(done)
 
         # Predictions
         while self._child.is_alive():
@@ -213,12 +217,15 @@ def _consume_events_inner(self) -> None:
                 if not done:
                     break
 
+            # We capture the predict future and then reset state before
+            # completing it, so that we can immediately accept work.
+            result = self._result
             self._predict_payload = None
             self._predict_start.clear()
-            self._result.set_result(done)
             self._result = None
             self._state = WorkerState.READY
             self._allow_cancel = False
+            result.set_result(done)
 
         # If we dropped off the end off the end of the loop, it's because the
         # child process died.