Rename the query system's `JobOwner` to `ActiveJobGuard`, and include `key_hash` `JobOwner` appears to have had more responsibilities in the past, but nowadays it's just a stack-guard object used by `execute_job` to poison the query state for the current key if some inner part of query execution panics. The new name and comments should hopefully be clearer about its (limited) role. I have also included a follow-up change that stores both the key and its previously-computed hash in the guard, instead of just the key. This avoids having to pass the key to `complete`, and avoids having to recompute the hash in `drop`. r? nnethercote (or compiler)