Implement graceful shutdown for the peer set

[teor2345]

Motivation

The peer set can keep running, even when some of its background tasks have shut down.
But when Zebra starts to shut down, we want to cancel all the tasks quickly.

This should speed up some tests, and avoid test failures.
Quick shutdowns also make Zebra easier to use.

This is unexpected work in sprint 23.

Solution

• when one peer set task shuts down, shut the rest of them down

Review

@oxarbitrage can review this low priority PR.

Reviewer Checklist

• Code makes sense
• Existing tests pass

Follow Up Work

• Design and implement graceful shutdown for Zebra #1678

[oxarbitrage]

Code looks good, thank you for adding documentation too.