Improve appservice handler to send only the most recent read receipts when no stream_id is stored. (#8744)

* Make this line debug (it's noisy)

* Don't include from_key for presence if we are at 0

* Limit read receipts for all rooms to 100

* changelog.d/8744.bugfix

* Allow from_key to be None

* Update 8744.bugfix

* The from_key is superflous

* Update comment

synapse/handlers/receipts.py

@@ -158,7 +158,8 @@ class ReceiptEventSource:
         if from_key == to_key:
             return [], to_key
 
-        # We first need to fetch all new receipts
+        # Fetch all read receipts for all rooms, up to a limit of 100. This is ordered
+        # by most recent.
         rooms_to_events = await self.store.get_linearized_receipts_for_all_rooms(
             from_key=from_key, to_key=to_key
         )