Avoid using message-based IO in the plaintext protocol

Can reuse the machinery in `secio` but need to generalize the
"buffering" there

libp2p/security/base_session.py

@@ -1,7 +1,6 @@
 from typing import Optional
 
 from libp2p.crypto.keys import PrivateKey, PublicKey
-from libp2p.io.msgio import MsgIOReadWriter
 from libp2p.peer.id import ID
 from libp2p.security.secure_conn_interface import ISecureConn
 
@@ -14,7 +13,6 @@ class BaseSession(ISecureConn):
 
     local_peer: ID
     local_private_key: PrivateKey
-    conn: MsgIOReadWriter
     remote_peer_id: ID
     remote_permanent_pubkey: PublicKey
 
@@ -22,27 +20,14 @@ class BaseSession(ISecureConn):
         self,
         local_peer: ID,
         local_private_key: PrivateKey,
-        conn: MsgIOReadWriter,
         peer_id: Optional[ID] = None,
     ) -> None:
         self.local_peer = local_peer
         self.local_private_key = local_private_key
         self.remote_peer_id = peer_id
         self.remote_permanent_pubkey = None
-
-        self.conn = conn
         self.initiator = peer_id is not None
 
-    async def write(self, data: bytes) -> int:
-        await self.conn.write(data)
-        return len(data)
-
-    async def read(self, n: int = -1) -> bytes:
-        return await self.conn.read(n)
-
-    async def close(self) -> None:
-        await self.conn.close()
-
     def get_local_peer(self) -> ID:
         return self.local_peer