Figuring out how to correctly limit updates from this stream without dropping
entries is far more complicated than just counting the number of rows being
returned. We need to consider each query separately and, if any one query hits
the limit, truncate the results from the others.
I think this also fixes some potentially long-standing bugs where events or
state changes could get missed if we hit the limit on either query.
The general idea here is to get rid of the type: ignore annotations on all of the current_token and update_function assignments, which would have caught #7290.
After a bit of experimentation, it seems like the least-awful way to do this is to pass the offending functions in as parameters to the Stream constructor. Unfortunately that means that the concrete implementations no longer have the same constructor signature as Stream itself, which means that it gets hard to correctly annotate STREAMS_MAP.
I've also introduced a couple of new types, to take out some duplication.
This changes the replication protocol so that the server does not send down `RDATA` for rows that happened before the client connected. Instead, the server will send a `POSITION` and clients then query the database (or master out of band) to get up to date.
* Port synapse.replication.tcp to async/await
* Newsfile
* Correctly document type of on_<FOO> functions as async
* Don't be overenthusiastic with the asyncing....
