Faster joins: support sending messages from clients during the half-joined state

[richvdh]

This is theoretically simple. Currently we are blocking on calculating whether there are remote servers we need to send to, but we have a separate list of servers, so we just need to update the code to use it

Subtasks

• Faster joins: Creating events from local users: Track the set of forward safe chain events and use them as prev_events #14057
• Faster joins: Creating events from local users: Don't block. Send events to the separate list of servers. #14058
• Faster joins: Creating events from local users: Answer queries over federation where we are obligated to answer #14059

[squahtx]

We'll probably want to test that remote Synapse instances can accept the events sent out while half-joined too.

As Erik notes, we have to respond to get_missing_events, among other things. There are also paths where a remote Synapse instance will request /state_ids or /state (which the half-joined server cannot answer!) when receiving an event with unknown prev events.

[reivilibre]

As part of #13288 (comment), I've suggested that, whilst a room is partial-stated, we keep track of (or otherwise know how to find out) which servers we've sent some events to and then answer relevant /get_missing_events queries for the prev_events of the events that we send, otherwise the remote homeserver has reason to reject the event. I suggest that we then don't answer in other cases, as we don't definitively know the set of servers in the room and so we can't necessarily know if the server is allowed to fetch events from us.

[reivilibre]

(assigning self because I have been thinking of ideas with Rich about some of the subtleties of sending PDUs and have some things to write up at the very least)