Throttle validation separately from other compaction

Validation compaction is fairly ressource intensive. It is possible to throttle it with other compaction, but there is cases where you really want to throttle it rather aggressively but don't necessarily want to have minor compactions throttled that much. The goal is to (optionally) allow to set a separate throttling value for validation.

PS: I'm not pretending this will solve every repair problem or anything.