Epic: limit cache validation to absolute necessity

[efiop]

We validate cache too much, almost every operation (status/repro/checkout/etc) does it every time and even though they don't rehash the files, it still takes a lot of time to validate cache for large datasets. This is mostly a leftover from our old hardlink/symlink defaults.

• introduce dvc cache/remote check (or some other name)
• don't validate on diff
• don't validate on checkout (checkout: don't validate cache #9572)
• don't validate on status (unless explicitly asked?)
• don't validate on repro
• ...

[dberenbaum]

@efiop Do we mention that it happens in the docs anywhere? For 3.0, I guess we can mention in the release notes that we will stop guaranteeing this behavior for performance reasons, and then we can keep working on it after release. WDYT?