Start adding a server (aka Netback)

I think we should probably copy the style of Vchan, ie have a

Endpoint.Make(MEMORY)(EVENTS)(CONFIGURATION) : sig
  include V1_LWT.FLOW ...

  val server: ... -> t Lwt.t
  val client: ... -> t Lwt.t
end

Signed-off-by: David Scott <dave.scott@citrix.com>