MINOR: flt-http-comp: Use compression options as filter config

MINOR: flt-http-comp: Use compression options as filter config

All comp filter configurations left their ->conf pointer NULL: the request
and response compression filters resolved the proxy options at runtime,
through strm_fe(s)->comp and s->be->comp, so the pointer was never used by
the stream callbacks.

Now store the proxy's compression configuration (px->comp) in the filter
configurations, for the explicit "filter compression", "filter comp-req" and
"filter comp-res" declarations as well as for the filters implicitly
declared by check_implicit_http_comp_flt(). The compression options become
the actual configuration of these filters, like the decompression filters
already do with their direction.

No stream callback is changed: the runtime resolution through the frontend
and backend proxies is preserved, since the options of both proxies are
combined to compress a message, with a priority to the backend ones. But the
attach callback can now rely on FLT_CONF() to know whether a filter can be
useful, and the http-comp filter instances will set the same pointer.

The configuration remains owned by the proxy and released with it: the
compression filters have no deinit callback freeing ->conf.