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/helpers.py

@@ -1,4 +1,5 @@
 """Various helpers not related to the Telegram API itself"""
+import asyncio
 import os
 import struct
 from hashlib import sha1, sha256
@@ -85,6 +86,22 @@ def retry_range(retries):
         yield 1 + attempt
 
 
+async def _cancel(log, **tasks):
+    """
+    Helper to cancel one or more tasks gracefully, logging exceptions.
+    """
+    for name, task in tasks.items():
+        if not task:
+            continue
+
+        task.cancel()
+        try:
+            await task
+        except asyncio.CancelledError:
+            pass
+        except Exception:
+            log.exception('Unhandled exception from %s after cancel', name)
+
 # endregion
 
 # region Cryptographic related utils