MINOR: flt-decomp: add support for HTTP decompression

MINOR: flt-decomp: add support for HTTP decompression

Following the previous patch, now we make use of the HTX API to support
HTTP decompression (when proxy is in "mode http"). The decompression
filter works in the similar way as the compression one, except that it
performs decompression instead of compression. As such, we can now use
HAproxy to perform end to end compression: compress input request,
forward it to a distant haproxy node, and let the other haproxy node
decompress request before forwarding it to servers, then do the opposite
operation for the response. This may enable bandwidth optimization when
it matters and the CPU overhead that comes with it is acceptable (check
out libslz writeups to know more about the performance impact).