This requires a RUSTFLAG to be set before hand for rayon to build.
This brakes a lot of tools like rls and clippy by default and
require special configs for itnegration.
Additionally, rayon_futures is still 0.1 and not much work seem
to have gone into it. Ideally it should be replased with the tokio
runtime/threadpool.
This is never used anywhere else apart from the testsuite. Instead
of ignoring etags we should instead not save them if the feed does
not return 200 or 304. See #64.
