fix(jobs): stale_timeout=0 means no grace period

[dimitri-yatsenko]

refresh() removes a job whose key has left key_source once the job is older than stale_timeout, a grace period for a key_source that is still being filled. stale_timeout=0 skipped that check instead, so there was no way to ask for the cleanup now: refresh(stale_timeout=0) left every such job in place, and so did populate() with config.jobs.stale_timeout = 0.

0 now means no grace period: on that refresh, every job whose key is no longer in key_source is removed, whatever its age, unless it is marked ignore. That includes reserved jobs, and success records of rows that remain after their key left key_source, which a nonzero stale_timeout keeps for that many seconds. stale_timeout=None still means the configured value (config.jobs.stale_timeout, 3600 seconds by default), so a refresh that does not pass it keeps the grace period. The setting's description, "Seconds before pending job is checked for staleness", now describes the grace period and what 0 does.

Skipping the check is no longer expressible as 0; a long stale_timeout keeps such jobs for as long as it says.

The tests delete upstream rows and check, on MySQL and PostgreSQL, that a default refresh keeps the pending jobs of the deleted keys, that refresh(stale_timeout=0) removes them, and that populate() with config.jobs.stale_timeout = 0 removes them instead of trying to compute them. Each fails without the change. They replace the test that held 0 to its old meaning.

Closes #1577.

Stacked on #1595: its base is fix/jobs-refresh-restriction until that merges.