Remove ackable_work; ack immediately instead (#1552)

A long time ago, one task accumulated ackable jobs into
`Downstairs::ackable_work` and a different task periodically read out
that set and sent acks back to the guest.

The tasks have long since been consolidated, but the `ackable_work` set
remained.

This PR removes the `ackable_work` set, acking jobs immediately when
they are ready. It's a significant LOC simplification, because it's
removing intermediate state (which previously had to be checked in unit
tests).

There's one subtlety in handling live-repair: previously, the
live-repair check would look in `ackable_work` to see if the next job
was ackable, which indicates that live-repair can continue. Now, we
update the `LiveRepairState` directly in `Downstairs::ack`, storing a
result for the pending live-repair job.