scheduler: Separate scheduling save code now tasks from the recurrent tasks

It happens that the recurrent tasks get stuck. It has been worked around but the main issue is not yet understood.

Given the save code now expectancy "make it reliable and fast", they must be scheduled independently from the rest. That should alleviate the current recurring pattern of the apparent "stuckness" of the save code now [1]

Implementation wise, that means having another scheduler runner separated from the current one whose concern would be to deal with save code now only. The actual one being modified to no longer grab tasks for the save code now.

• [1] It is stuck but it's not only the save code now... It's mostly all tasks.

Related to T3361

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Migrated from T3367 (view on Phabricator)