refactor: Change MuxRPC.__await__ MuxRPC.process_messages

`__await__` can be a bit dangerous in the world of `asyncio`.

ssb/muxrpc.py

@@ -155,11 +155,15 @@ class MuxRPCAPI:
         self.handlers = {}
         self.connection = None
 
-    async def __await__(self):
+    async def process_messages(self):
+        """Continuously process incoming messages"""
+
         async for req_message in self.connection:
             body = req_message.body
+
             if req_message is None:
                 return
+
             if isinstance(body, dict) and body.get("name"):
                 self.process(self.connection, MuxRPCRequest.from_message(req_message))