This substituter connects to a remote host, runs nix-store --serve
there, and then forwards substituter commands on to the remote host and
sends their results to the calling program. The ssh-substituter-hosts
option can be specified as a list of hosts to try.

This is an initial implementation and, while it works, it has some
limitations:

* Only the first host is used
* There is no caching of query results (all queries are sent to the
  remote machine)
* There is no informative output (such as progress bars)
* Some failure modes may cause unhelpful error messages
* There is no concept of trusted-ssh-substituter-hosts

Signed-off-by: Shea Levy <shea@shealevy.com>