Refactor MsgIOReadWriter

- Abstract it as `MsgReadWriter`
- `MsgIOReadWriter` as a subclass of `MsgReadWriter`

libp2p/io/abc.py

@@ -32,3 +32,23 @@ class ReadWriter(Reader, Writer):
 
 class ReadWriteCloser(Reader, Writer, Closer):
     pass
+
+
+class MsgReader(ABC):
+    @abstractmethod
+    async def read_msg(self) -> bytes:
+        ...
+
+    @abstractmethod
+    async def next_msg_len(self) -> int:
+        ...
+
+
+class MsgWriter(ABC):
+    @abstractmethod
+    async def write_msg(self, msg: bytes) -> None:
+        ...
+
+
+class MsgReadWriter(MsgReader, MsgWriter):
+    pass