Prepare v4.0.1

[dinosaure]

/cc @craigfe, @hannesm & @avsm. It's a minor release which does not integrate #395 - So I will not merge this PR but I will just produce the tag and the tarball needed for the release. The minor release is needed because:

1. I'm not sure that we want a release which update the interface (and imply several others releases by cascading)
2. Some PR remains and should be a part of the next release (specially use close-notify for unidirectional shutdown #319 & Add openssl_overrides to conduit context #390)

I will cherry-pick the commit which has the changelog update.

[hannesm]

Thanks @dinosaure, this looks good to me.

[dinosaure]

The failure on ocaml-ci is unrelated and it's about core/async. Let's do the release.