Introduce the server functionality in the kiwix-lib.

This code is mainly copied from kiwix-tools.

But :
- Move all the response thing in a new class Response.
- This Response class is responsible to handle all the MHD_response
  configuration. This way the server handle a global object and do
  no call to MHD_response*
- Server uses a lot more the templating system with mustache.
  There are still few regex operations (because we need to
  change a content already existing).
- By default, the server serves the content using the id as name.
- Server creates a new Searcher per request. This way, we don't have
  to protect the search for multi-thread and we can do several search
  in the same time.
- search results are not cached, this will allow future improvement in the
  search algorithm.
- the home page is not cached.
- Few more verbose information (number of request served, time spend to
  respond to a request).

TOOD:
 - Readd interface selection.
 - Do Android wrapper.
 - Remove KiwixServer (who use a external process).
 -

include/meson.build

@@ -11,6 +11,7 @@ headers = [
   'entry.h',
   'searcher.h',
   'search_renderer.h',
+  'server.h',
   'kiwixserve.h',
   'name_mapper.h'
 ]