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/network/mtprotosender.py

@@ -749,14 +749,17 @@ class _ContainerQueue(asyncio.Queue):
                 isinstance(result.obj, MessageContainer):
             return result
 
+        size = result.size()
         result = [result]
         while not self.empty():
             item = self.get_nowait()
-            if item == _reconnect_sentinel or\
-                    isinstance(item.obj, MessageContainer):
+            if (item == _reconnect_sentinel or
+                isinstance(item.obj, MessageContainer)
+                    or size + item.size() > MessageContainer.MAXIMUM_SIZE):
                 self.put_nowait(item)
                 break
             else:
+                size += item.size()
                 result.append(item)
 
         return result