Add delim_encode and delim_read

- Add `StreamCommunicator` and `RawConnectionCommunicator`, read/write
messages with delim codec, with `IMuxedStream` and `IRawConnection`
respectively.
- Use it in `Multiselect` and `MultiselectClient`.

libp2p/security/secure_conn_interface.py

@@ -14,6 +14,8 @@ Relevant go repo: https://github.com/libp2p/go-conn-security/blob/master/interfa
 
 
 class AbstractSecureConn(ABC):
+    conn: IRawConnection
+
     @abstractmethod
     def get_local_peer(self) -> ID:
         pass