feat(task-runner): stop running tasks when the process is interrupted

[ddeboer]

A task outlives the process that started it: a native task runs in a process group of its own, so a Ctrl-C never reaches it, and a container runs under the Docker daemon, which hears nothing of the process at all. Interrupting a SPARQL Anything conversion therefore left JVMs running and a sparql-anything-* directory behind.

@lde/task-runner now has ChildTasks, for implementing a runner – a runner’s users never see it. While any runner has a task going, the process listens for SIGINT and SIGTERM, stops every task the way that runner’s stop() does, and then ends as it would have without the listening: with the signal’s exit status (130, 143), or however a listener of the process’s own ends it. Nothing listens while no task is going, so an idle runner leaves the process’s signal handling as it found it.

What that binding covers:

• Every process NativeTaskRunner spawns and every container DockerTaskRunner starts, so QLever’s index build and server come along too.
• The listening goes in front of any listener the process already had, so what stop() does before its first await – sending a native task’s process group its SIGTERM – happens even when that listener ends the process at once. A container’s stop is a request to the daemon, so it carries no such guarantee; the docs page says so rather than overpromising.
• A task that starts while the tasks a signal found are being stopped is stopped too, and the process waits for it – the sweep is a snapshot, and a chunk spawning just behind it would otherwise be missed.
• A signal the process survives leaves the listening ready again, so tasks started afterwards are bound just the same.
• The state is shared between copies of this package in one process. It is a plain dependency, so version skew can duplicate it, and two listeners each waiting for the other to end the process would hang a Ctrl-C.
• A second signal while the tasks are being stopped ends the process at once, so a task that will not stop cannot hold it hostage.

SparqlAnythingConverter drops its own signal handling for this: its run directory goes on exit, synchronously, and that listener is only removed once the normal cleanup has actually finished – an interrupted run could otherwise end the process between the two and leave the directory behind.

Fix #831