Introduce config for default task duration

[gsailer]

This introduces the key distributed.scheduler.task-duration in the Dask config to configure a default task duration for all tasks, when there is no otherwise configured value available.

That is useful in cases of very long-running tasks to tune the occupancy of the cluster (see #3516 and #3627 )

[quasiben]

Thanks @sublinus . Looks reasonable to me and definitely appreciate the test! I am going to wait until the end of the day to merge should other maintainers want to comment

[TomAugspurger]

Looks nice. My only question would be on the name. Do we like task-duration or should we call out unknown-task-duration?

[mrocklin]

Should this have a non-null value? What do we do currently?

So far we've been limiting comments for configuration values to be a bit shorter and be on the same line. If we choose to change convention here and have much larger comments then we should probably do that across the file. For now I recommend that we try to make the comment more brief if we can.

[gsailer]

Thanks for all your comments. The question regarding the non-null value just pointed me to scheduler.get_task_duration, which is where I think this unknown-task-duration should be used instead of in the TaskPrefix as an average.
Currently the default used for tasks with unknown task durations is 0.5s, which is now reflected in the config.
I totally see your point regarding the comment style.

[gsailer]

The partial CI failure seems to be unrelated here.

[quasiben]

@sublinus i think you are probably right but I restarted the tests job to be sure.

[mrocklin]

Two small comments. Otherwise this looks fine to me.

[gsailer]

The build shows as in progress even though GH Actions and Travis ran through fine. It looks like the notify about the successful build from Travis to Github failed.

[mrocklin]

Thanks @sublinus ! This is in