MINOR: config: Support per-proxy and per-server deinit functions callbacks

MINOR: config: Support per-proxy and per-server deinit functions callbacks

Most of times, when any allocation is done during configuration parsing because
of a new keyword in proxy section or on the server line, we must add a call in
the deinit() function to release allocated ressources. It is now possible to
register a post-deinit callback because, at this stage, the proxies and the
servers are already releases.

Now, it is possible to register deinit callbacks per-proxy or per-server. These
callbacks will be called for each proxy and server before releasing them.