Replace establish_server by establish_server_safe

[aantron]

Lwt_io.establish_server_safe closes channels automatically and was introduced in #258. I looked at users of the original establish_server in OPAM (as of today), and none would be negatively affected by closing channels automatically. Hopefully, this also holds true of most or all non-OPAM users.

The signatures are incompatible, so code would have to be adjusted:

val establish_server_safe :
  ?fd : Lwt_unix.file_descr ->
  ?buffer_size : int ->
  ?backlog : int ->
  Unix.sockaddr -> (input_channel * output_channel -> unit Lwt.t) -> server

val establish_server :
  ?fd : Lwt_unix.file_descr ->
  ?buffer_size : int ->
  ?backlog : int ->
  Unix.sockaddr -> (input_channel * output_channel -> unit) -> server