Stagger key rotation when unwinding invalidated invitation

[HerbCaudill]

Multiple admins could simultaneously see that there are pending key rotations. It might make sense to add a random delay before rotating keys, or to come up with another way to ensure that these don't pile up.

This happens in Team.checkForPendingKeyRotations

Test plan:

• Builds on the test "unwinds an invalidated admission" in sync.test.ts
• Instead of just Alice, there are multiple admins
• Assert that the team keys are only rotated once