Factor out django_utils.command

[benjaoming]

KA Lite should have a simple and unified mechanism for starting other processes. The django_utils.command is a mess.

We know that this stuff has attracted lots of problems, so let's assemble our best practices into one place and ensure stuff like #3704 and #4115 stops happening.

Cherrypy is running a thread pool while KA Lite is active.

The python-packages/fle_utils/django_utils/command.py is in a sad state, we could ideally abandon it in favor of a simple approach.

Let's discuss a simple and low-risk for 0.14.

One of the sad things in python-packages/fle_utils/django_utils/command.py is that it declares call_command_subprocess which uses the subprocess library and then REDECLARES call_command_subprocess as a function that uses multiprocessing :/

One much simpler approach would be to use normal threads that are signalled to shutdown by the main process in the same fashion that bin/kalite runs cronserver_blocking and tells it to exit. This reduces risks of process leaks clogging memory and leaked processes accessing the same files as newer processes.

[benjaoming]

This can be fixed once #4892 is solved