CacheConfigs are always enabled; clarify directory validation

[jder]

This makes two small changes to the CacheConfig/Cache system:

First, the comments referred to enabled/disabled caches, which is no longer a thing after #10665, so I've removed this. There were also some docs about panics in these cases (which are also now impossible) which I removed.

Second, CacheConfig::default()/CacheConfig::new() produced configs which would panic if you asked for their directory without first calling validate, but validate is not public API. So instead we return an Option from CacheConfig::directory() and move the panic to Cache::directory() and WorkerThread::directory() which should be impossible to trigger (since validate is called by Cache::new). We could alternatively make a ValidatedCacheConfig or something which is returned from validate and store/pass that around, but it seemed a bit overkill to me.

cc @benbrandt who was working on this recently in case I'm missing something.

[alexcrichton]

Thanks!

[github-actions]

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