Allow setting task time limits

[Vectorial1024]

Along with this, also include a way to catch "process timed out" events to let the task creator specify on-time-out actions.

[Vectorial1024]

Experiments show that Process::timeout() is ignored when using Process::start(): the started process will still run until it finishes. We need custom handling of task time limits.

[Vectorial1024]

To clarify, we are talking about real time, not CPU time. This will include all the time spent on sleeping and waiting that normally may not be counted by PHP (Unix). This has irl value where the overall timing is important.

[Vectorial1024]

Blunder for forgetting to link the issue/PR first; closing as done.