Use the new in-memory entity cache

This should avoid a disk access every time an input entity
is needed, which is very often. Another step for #1141.

telethon/tl/custom/draft.py

@@ -64,9 +64,8 @@ class Draft:
         """
         if not self._input_entity:
             try:
-                self._input_entity =\
-                    self._client.session.get_input_entity(self._peer)
-            except ValueError:
+                self._input_entity = self._client._entity_cache[self._peer]
+            except KeyError:
                 pass
 
         return self._input_entity