BuildStream now has artifact cache sharing

[Tristan Van Berkom <tristan vanberkom codethink co uk>]

Just wanted to share that artifact cache sharing is now landed.

In fact it landed over a week ago, but the initial implementation was
not working at all for high latency connections (pushing to Manchester
from Seoul for example) and was all around slow on push in general.

As I understand, it's a hard requirement for Baserock moving forward to
using BuildStream.

So, the documentation[0] now contains the section "Artifact Caches"[1]
which should outline everything anyone needs to know about setting up
an artifact cache for BuildStream.

Cheers,
    -Tristan

[0]: https://buildstream.gitlab.io/buildstream/
[1]: https://buildstream.gitlab.io/buildstream/artifacts.html#artifacts