Improve how the task scheduler deals with tasks that already have a
worker assigned to them:
- When a Worker asks for a task, and there is already an active task
assigned to it, always return that task.
- Otherwise, never allow scheduling of active tasks, as those are
already being run by another worker. If this is not the case, their
status should change to queued/failed, instead of handling the
situation in the task scheduler.
- Apart from the assigned-and-active case above, ignore task's worker ID
when scheduling tasks. If the status is 'queued' or 'soft-failed', the
task's worker ID just indicates who ran the task last.
The add-on code was copy-pasted from other addons and used the GPL v2
license, whereas by accident the LICENSE text file had the GNU "Affero" GPL
license v3 (instead of regular GPL v3).
This is now all streamlined, and all code is licensed as "GPL v3 or later".
Furthermore, the code comments just show a SPDX License Identifier
instead of an entire license block.
The root cause was a 2nd `context.Context()` that was used in
`constructTestJob()`, which cancelled when that function returned.
The cancellation of the context caused an interrupt in the SQLite
driver, which got into a race condition and could cause an interrupt on
a subsequent database query.
The on-disk database that was used before caused issues with tests running
in parallel. Not only is there the theoretical issue of tests seeing each
other's data (this didn't happen, but could), there was also the practical
issue of one test running while the other tried to erase the database file
(which fails on Windows due to file locking).
