Make the Connection a proper ABC (#509)

telethon/network/connection/tcpintermediate.py

@@ -0,0 +1,23 @@
+import struct
+
+from .tcpfull import ConnectionTcpFull
+
+
+class ConnectionTcpIntermediate(ConnectionTcpFull):
+    """
+    Intermediate mode between `ConnectionTcpFull` and `ConnectionTcpAbridged`.
+    Always sends 4 extra bytes for the packet length.
+    """
+    def connect(self, ip, port):
+        result = super().connect(ip, port)
+        self.conn.write(b'\xee\xee\xee\xee')
+        return result
+
+    def clone(self):
+        return ConnectionTcpIntermediate(self._proxy, self._timeout)
+
+    def recv(self):
+        return self.read(struct.unpack('<i', self.read(4))[0])
+
+    def send(self, message):
+        self.write(struct.pack('<i', len(message)) + message)