Bug 2004406 - Use multiprocessing for the MWU test version, and clean up the API code.

[gmierz]

This patch adds multiprocessing capabilities for the MWU test version processing. It should provide some significant performance gains for API requests from PerfCompare. Local runs for a worst case scenario suggest improvements from 40s to 18s for a request like this one when silverman/KDE was enabled: https://treeherder.mozilla.org/api/perfcompare/results/?base_repository=try&base_revision=bddcdb7187bbbae40da023ff24315d5e499dc0a3&new_repository=try&new_revision=7a30b2c44b19a3576f8c87c56a001792d549e5e0&framework=13&no_subtests=true&replicates=true&test_version=mann-whitney-u

When silverman/KDE is disabled, we still see a pretty big improvement going from 4s without multiprocessing to ~2.5s with multiprocessing.