BUG/MEDIUM: filters: Fix data filtering when data are modified

BUG/MEDIUM: filters: Fix data filtering when data are modified

Filters can alter data during the parsing, i.e when http_data or tcp_data
callbacks are called. For now, the update must be done by hand. So we must
handle changes in the channel buffers, especially on the number of input bytes
pending (buf->i).
In addition, a filter can choose to switch channel buffers to do its
updates. So, during data filtering, we must always use the right buffer and not
use variable to reference them.

Without this patch, filters cannot safely alter data during the data parsing.