Implement MSC4023: Thread ID for 2nd order-relation in Synapse

[matrixbot]

This issue has been migrated from #15701.

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The proposal matrix-org/matrix-spec-proposals#4023 requires an implementation in Synapse.

This issue is part of the ongoing effort to fix stuck notifications in Element Web. As mentioned in @neilisfragile's engineering newsletter this week, we had to skip the last Element Web release because one of our client-side fixes has unhidden a larger battery of other issues that we have since been working on.

The biggest remaining issue we've narrowed things down onto this week is element-hq/element-web#25450. We've put in a client-side fix for this but the excess requests to fetch the unknown events have a performance impact. The size of the latter is hard to gauge but in our tests it was as bad as 5x in extreme cases.

After testing on today's nightly, we believe the general experience is good enough to unblock our release but preferably we'd like to limit the time window where performance is negatively impacted by leveraging server-side support through MSC4023.