Streamline download with multiprocessing and more parallelism

[rtibbles]

Summary

• Adds better handling for multiprocessing task workers for the threading manager used in resource import
• Adds required fixes for full worker multiprocessing
• Restricts multiprocessing to Python 3.7 plus because of initializer kwargs support for pool executors
• Increases the potential parallelism for resource download tasks by increasing the number of max workers to 50, and increasing the requests connection pool size to match

References

Potential future improvement for #13680

Reviewer guidance

This mostly works, but I have observed issues with tasks stalling, and when that happens Kolibri also taking a long time to shut down. Until we can isolate what is causing this (which presumably have the same root cause), multiprocessing workers cannot be the default.

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.18.2a0.dev0_git.12.gb511fe64.pex
Windows Installer (EXE)	kolibri-0.18.2a0.dev0+git.12.gb511fe64-windows-setup-unsigned.exe
Debian Package	kolibri_0.18.2a0.dev0+git.12.gb511fe64-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.18.2a0.dev0+git.12.gb511fe64.dmg
Android Package (APK)	kolibri-0.18.2a0.dev0+git.12.gb511fe64-0.1.5-debug.apk
Raspberry Pi Image	kolibri-pi-image-0.18.2a0.dev0+git.12.gb511fe64.zip
TAR file	kolibri-0.18.2a0.dev0+git.12.gb511fe64.tar.gz
WHL file	kolibri-0.18.2a0.dev0+git.12.gb511fe64-py2.py3-none-any.whl