Don't share full key history for RTC per-participant encryption

[hughns]

Instead we only share what is needed.

This PR also records stats for how many keys have been sent/received and age of those received.

Checklist

• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• Sign-off given on the changes (see CONTRIBUTING.md).

[robintown]

I don't fully understand what the 'total age' statistic is there for (wouldn't it be more useful to know say, the max, min, and average age?) but as long as you have a use case for it in mind, this looks fine.

[hughns]

I don't fully understand what the 'total age' statistic is there for (wouldn't it be more useful to know say, the max, min, and average age?) but as long as you have a use case for it in mind, this looks fine.

The total age is there to calculate the average age later on. I've improved the comment to explain this. 👍