Replace chronograph with threaded task queue

[benjaoming]

Summary

Replacing #2891

It should be fairly easy to create a task consumer thread with workers in subthreads.

Number of workers will be configurable.

Communication will be file-based by means of well-know text in files in KALITE_HOME. The database-backed progress stuff was not the way to go IMO, so replacing with something simpler. For instance, it meant that tasks failed when database wasn't created or when accessed to much.

Marking as release blocker because we've had too many problems with video downloads in previous releases. Going to create this as a proper implementation that could be used for Kolibri.

It won't necessarily replace the entire chronograph because I'm not going to force the replacement of sync jobs as a requirement for having something merged before the release.

Fixes / related

#4880
#4817
#4789
#4781
#4739
#4759
#3310
#5231
#5561
#5575