Clean up overseer modules slightly and provide a throttling support method

Summary:
Ref T12298. Make it slightly easier to write overseer modules:

• You don't need to implement shouldReloadDaemons(), since you can now just implement shouldWakePool() and get useful behavior.
• It looks like most modules want to do checks only every X seconds, where X is at least somewhat module/use-case dependent, so add a helper for that.

Test Plan: Ran bin/phd debug pull --trace, saw config check fire properly every 10 seconds after updating it to use shouldThrottle() in the next change.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12298

Differential Revision: https://secure.phabricator.com/D17539