Track command processing as a background process (#7879)

I'm going to be doing more stuff synchronously, and I don't want to lose the
CPU metrics down the sofa.

stubs/txredisapi.pyi

@@ -22,6 +22,7 @@ class RedisProtocol:
     def publish(self, channel: str, message: bytes): ...
 
 class SubscriberProtocol:
+    def __init__(self, *args, **kwargs): ...
     password: Optional[str]
     def subscribe(self, channels: Union[str, List[str]]): ...
     def connectionMade(self): ...