Support latest Async and improve IPv4/6 resolution logic

• Favour resolving over IPv4 instead of IPv6, if both are available
  and one has to be chosen. (#245 via @rixed)
• Fix some warnings with the dummy Lwt_unix_ssl module.
• Add a direct dependency on xenstore for mirage-conduit.
• Support latest Async v0.10.0 interfaces (no more Async.Std).