conf: extract lazily computed and cached configuration

Created by: keegancsmith

In a few places we want to compute a value from the config. We don't need to do it on start, but only when it is first used. So we can use a mix of sync.Once to kick off conf.Watch + some synchronization like a Mutex or atomic Value. This commit introduces a helper to simplify this pattern.

This pattern I wanted to use in some new code I was writing. I realised we used it in some other places so I am putting the pattern in the conf package. One benefit is the sites that use this pattern are now much clearer.

This commit only updates one of the call sites which use this pattern.