Use Rust Mercurial parts by default
We've been running the Rust native code on this instance and on Octobus' for a while now, with no problems.
It is time to activate it by default in Omnibus packages and the Docker image. It will still be possible to disable, see !20 (merged).
Ideally this should have been done for 0.20.0rc1, but we'll do it before the final release.