MAJOR: stats: Update HTTP stats applet to handle its own buffers

MAJOR: stats: Update HTTP stats applet to handle its own buffers

The HTTP stat applets and all internal functions was adapted to use its own
buffers instead of the channels ones. The CLI part was not refactored yet,
thus there are still some access to channels in the file. But for the HTTP
part, we no longer use the channels at all.

To do so, the HTTP stats applet now uses default .rcv_buf and .snd_buf
callback function. In addition, it sets appctx flags instead of SE ones.