Support read(n=-1)

Now, `n=-1` indicates that we want to read until EOF. However, now we
only read until we have no new message.

libp2p/stream_muxer/mplex/mplex.py

@@ -1,5 +1,5 @@
 import asyncio
-from typing import Dict, Tuple, Optional
+from typing import Dict, Optional, Tuple
 
 from multiaddr import Multiaddr