Avoid unnecessary copies when filtering private read receipts. (#12711)

A minor optimization to avoid unnecessary copying/building
identical dictionaries when filtering private read receipts.

Also clarifies comments and cleans-up some tests.

synapse/handlers/initial_sync.py

@@ -143,7 +143,7 @@ class InitialSyncHandler:
             to_key=int(now_token.receipt_key),
         )
         if self.hs.config.experimental.msc2285_enabled:
-            receipt = ReceiptEventSource.filter_out_private(receipt, user_id)
+            receipt = ReceiptEventSource.filter_out_private_receipts(receipt, user_id)
 
         tags_by_room = await self.store.get_tags_for_user(user_id)
 
@@ -449,7 +449,9 @@ class InitialSyncHandler:
             if not receipts:
                 return []
             if self.hs.config.experimental.msc2285_enabled:
-                receipts = ReceiptEventSource.filter_out_private(receipts, user_id)
+                receipts = ReceiptEventSource.filter_out_private_receipts(
+                    receipts, user_id
+                )
             return receipts
 
         presence, receipts, (messages, token) = await make_deferred_yieldable(