HTTP: new parser and main structures refactoring

This is the first patch of a big change that is coming, it does:

  - deprecate mk_request.[ch]
  - deprecate mk_method.[ch]
  - replace struct client_session by struct mk_http_session
  - replace struct session_request by struct mk_http_request
  - new HTTP parser mk_http_parser.c

At this moment the server 'compiles'. It now needs to be aware about
the new parser and also needs some cleanup on headers interfaces.

the fun begins..

Signed-off-by: Eduardo Silva <eduardo@monkey.io>