Improve Fluent translation cache
Improve Fluent translation cache

It was caching the `FluentLocalization` object, but that caused issues
in multithreaded environments (like the development server) due to
simultaneous attempts at iteration. This change caches loading the
Fluent resources which is the expensive work of parsing the files
and bundling the results into lists of resources.

Fix #8705

mozilla/bedrock,MichaelKohler/bedrock,ericawright/bedrock,pascalchevrel/bedrock,hoosteeno/bedrock,flodolo/bedrock,mozilla/bedrock,craigcook/bedrock,ericawright/bedrock,pascalchevrel/bedrock,ericawright/bedrock,alexgibson/bedrock,alexgibson/bedrock,pascalchevrel/bedrock,sylvestre/bedrock,craigcook/bedrock,hoosteeno/bedrock,ericawright/bedrock,hoosteeno/bedrock,craigcook/bedrock,flodolo/bedrock,flodolo/bedrock,sylvestre/bedrock,pascalchevrel/bedrock,mozilla/bedrock,sylvestre/bedrock,craigcook/bedrock,flodolo/bedrock,MichaelKohler/bedrock,MichaelKohler/bedrock,hoosteeno/bedrock,MichaelKohler/bedrock,alexgibson/bedrock,sylvestre/bedrock,alexgibson/bedrock,mozilla/bedrock