[MEDIUM] reorganized request handling to prepare for content-switching

[MEDIUM] reorganized request handling to prepare for content-switching

The filters are now iterated for FE, FI, BE.

Some grey areas remain :
  - uri_auth has been propagated to the backend, but in fact it
    should be checked at every level (fe, fi, be), depending
    where it is declared, and before the filters.

  - the HTTP method and URI should be stored and propagated everywhere
    they are used. For this, we would need to first apply filters to
    be aware of filter changes which affect them.

  - there seems to be no need anymore for hdr_idx[0] being empty.
    It may contain the start line, which will slightly improve
    performance and make the code easier to read.