Evict oldest OTKs if clients upload more than 500 keys

[kaylendog]

Motivation

In matrix-org/matrix-js-sdk#5501, clients can upload arbitrary numbers of one-time keys (up to several thousand) while forgetting about their private halves (Vodozemac keeps 100 locally), leading peers to claim OTKs and use them to send pre-key sessions the client cannot decrypt, preventing new Olm sessions from being established.

No server-side limit exists in Synapse, and the specification does not prevent us from forgetting about older keys as long as we guarantee only one peer can ever claim a particular key. This limit would mitigate some of the effects of OTK starvation, although it is best paired with the relevant JS SDK fix, matrix-org/matrix-js-sdk#5503.

Notes

Two important things worth considering before merging:

• This PR makes changes to the database schema;
• The OTK eviction task could start performing a lot of work when deployed to old/large homeservers, e.g. matrix.org - worth checking how many devices already exceed the 500 limit.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
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  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct (run the linters)

[kaylendog]

Closed in favour of #20162 which is both simpler and safer for pre-existing large homeservers.