rework async client handling
rework async client handling

- make async support persistent connections the http 1.1 default. no
  pipelining yet however.
- use the new body type when writing responses
- set the response "connection" header appropriately

more work left ahead:

- support keep-alive options like max connections/timeout
- share more code with lwt if possible

Drup/ocaml-cohttp,inhabitedtype/ocaml-cohttp,mor1/ocaml-cohttp,edwintorok/ocaml-cohttp,inhabitedtype/ocaml-cohttp,Drup/ocaml-cohttp,Drup/ocaml-cohttp,mor1/ocaml-cohttp,mor1/ocaml-cohttp,fdopen/ocaml-cohttp,edwintorok/ocaml-cohttp,marklrh/ocaml-cohttp,fdopen/ocaml-cohttp,inhabitedtype/ocaml-cohttp,fdopen/ocaml-cohttp,dsheets/ocaml-cohttp,dsheets/ocaml-cohttp,dsheets/ocaml-cohttp,marklrh/ocaml-cohttp,marklrh/ocaml-cohttp,edwintorok/ocaml-cohttp