tornado: Add transitional code for sender_delivery_email.

This issue was introduced in 54e357e154.

zerver/tornado/event_queue.py

@@ -821,6 +821,16 @@ def process_message_event(event_template: Mapping[str, Any], users: Iterable[Map
     @cachify
     def get_client_payload(apply_markdown: bool, client_gravatar: bool) -> Dict[str, Any]:
         dct = copy.deepcopy(wide_dict)
+
+        # Temporary transitional code: Zulip servers that have message
+        # events in their event queues and upgrade to the new version
+        # that expects sender_delivery_email in these events will
+        # throw errors processing events.  We can remove this block
+        # once we don't expect anyone to be directly upgrading from
+        # 2.0.x to the latest Zulip.
+        if 'sender_delivery_email' not in dct:  # nocoverage
+            dct['sender_delivery_email'] = dct['sender_email']
+
         MessageDict.finalize_payload(dct, apply_markdown, client_gravatar)
         return dct