Key backup restore API lacks sensible pagination

[richvdh]

The existing key backup restore API offers three ways to fetch backups:

• You can get the full backup at once, which is slow and impractical for huge backups (several 10s of minutes for big backups).
• It allows queries per session and in this case will require a lot of requests to get history.
• Finally it’s possible to get all keys for a given room but with no particular order. This can still take a long time and will retrieve old keys for history that is not even downloaded on the client.

A better API should provide access to keys for a given room with pagination, allowing a new login to decrypt recent history very quickly. This is particularly useful when using next gen clients that will do a lot of smart spidering to just bottleneck on the download of keys.