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.
SQLite can return `SQLITE_BUSY` errors when it's doing too many things at
the same time. This is now improved a bit by setting a 5-second timeout,
during which the SQLite driver will wait for the database to become
available. If that doesn't happen, Flamenco Manager will return a
`503 Service Unavailable` response so that the client knows to back off
a little.
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.
Instead of returning an error "error doing X", just return "doing X". The
fact that it's returned as an error object says enough about that it's
an error.
This also makes it easier to chain error messages, without seeing the
word "error" in every part of the chain.
