Avoid exceeding maximum container size

This issue would likely be triggered when automatically
merging multiple requests into a single one while having
their size exceed 1044456 bytes like SaveFilePartRequest.

This commit avoids such issue by keeping track of the
current size, and if it exceeds the limit, avoid merge.

telethon/tl/core/tlmessage.py

@@ -82,3 +82,6 @@ class TLMessage(TLObject):
             raise TypeError('Incoming messages should not be bytes()-ed')
 
         return self._body
+
+    def size(self):
+        return len(self._body)