Dune upgrade and socket fixes

Anil Madhavapeddy (2):
upgrade build to dune from jbuilder and update CI
upgrade opam metadata to 2.0 format

Yang Qian (1):
CA-289145: close socket if error occurs when using lwt connect

Add backwards compatibility ocamlfind packages

• add backwards-compat ocamlfind packages
  • xenstore_transport.unix
  • xenstore_transport.lwt