Sanchda/singleton watchers

[sanchda]

This is not ready for review, but if you wanna look, you can.

OK, so I wrote this on the plane ride from Paris without being able to compile it since my docker was broken, so it needs a lot of work, but the idea is to refactor our perf event management.

The reason for this: right now it's hard to extend things and we have a few upcoming needs

• Multiplex ringbuffers (handled in this PR, although globally)
• Scrape sample IDs (handled in this PR)
• Better management of multiple watchers (not handled here, but this should make it easier I hope???)

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