Make event persisters periodically announce position over replication. (#8499)

Currently background proccesses stream the events stream use the "minimum persisted position" (i.e. `get_current_token()`) rather than the vector clock style tokens. This is broadly fine as it doesn't matter if the background processes lag a small amount. However, in extreme cases (i.e. SyTests) where we only write to one event persister the background processes will never make progress.

This PR changes it so that the `MultiWriterIDGenerator` keeps the current position of a given instance as up to date as possible (i.e using the latest token it sees if its not in the process of persisting anything), and then periodically announces that over replication. This then allows the "minimum persisted position" to advance, albeit with a small lag.

synapse/storage/util/sequence.py

@@ -126,6 +126,8 @@ class PostgresSequenceGenerator(SequenceGenerator):
         if max_stream_id > last_value:
             logger.warning(
                 "Postgres sequence %s is behind table %s: %d < %d",
+                self._sequence_name,
+                table,
                 last_value,
                 max_stream_id,
             )