Revert disconnect() to be async again (#1133)

It's the only way to properly clean all background tasks,
which the library makes heavy use for in MTProto/Connection
send and receive loops.

Some parts of the code even relied on the fact that it was
asynchronous (it used to return a future so you could await
it and not be breaking).

It's automatically syncified to reduce the damage of being
a breaking change.

telethon/network/connection/tcpmtproxy.py

@@ -111,7 +111,7 @@ class TcpMTProxy(ObfuscatedConnection):
             await asyncio.sleep(2)
 
         if self._reader.at_eof():
-            self.disconnect()
+            await self.disconnect()
             raise ConnectionError(
                 'Proxy closed the connection after sending initial payload')