Improving performance of the build tool

[MMK21Hub]

Currently, the build tool takes quite a long time to run, especially when many versions are targeted. The main consequence of this is slower development of the pack, so I'm looking to improve the build times. Besides - more speed is more fun!

[MMK21Hub]

The longest part of the build process is actually validating each Fix object, because it often has to make multiple calls to the Mojira API (this takes about 20s). Fortunately, each fix is validated asynchronously - otherwise it would take even longer!

Actually running the transformers to generate the language files takes ~8s, so there's room for improvement there as well.