jobs: enter the paused state only after the resumer returns

Currently, the `paused-requested` -> `paused` state transitions happens when the job system cancels job's context rather than when the resumer actually returns. So, Resumer code could still be running even if the jobs table indicates the job is paused. This seems sad.

As @stevendanna notes: "The thing to think through is the semantics of the state transitions in cases where the owning node has become unavailable. You will never have a guarantee that the resumer isn't still running somewhere. So it is also probably worth thinking about whether we actually need to change the current semantics for anything other than me being occasionally annoyed by it."

Jira issue: CRDB-32824