Include thread information when sending receipts over federation. (#14466)

Include the thread_id field when sending read receipts over
federation. This might result in the same user having multiple
read receipts per-room, meaning multiple EDUs must be sent
to encapsulate those receipts.

This restructures the PerDestinationQueue APIs to support
multiple receipt EDUs, queue_read_receipt now becomes linear
time in the number of queued threaded receipts in the room for
the given user, it is expected this is a small number since receipt
EDUs are sent as filler in transactions.

synapse/handlers/receipts.py

@@ -92,7 +92,6 @@ class ReceiptsHandler:
                         continue
 
                     # Check if these receipts apply to a thread.
-                    thread_id = None
                     data = user_values.get("data", {})
                     thread_id = data.get("thread_id")
                     # If the thread ID is invalid, consider it missing.