We had a number of issues in practice with configuration providers where
things would race and as well as some unsafe threading/false-sharing.

This redesigns those components to avoid a number of issues in thread
safety.

There doesn't seem to be any regressions. However, it has pointed out
a few things that are/were broken in the flatpak configuration provider.
We will address those as part of a revamped build preferences that is
more pluggable (See #344 and #352).

Another piece that would be nice to apply on top of this is tracking
the last selected configuration so when restarting Builder we keep
the same config active (See #338).

Fixes #359