MINOR: flt-http-comp: Stop to explicitly loop on the proxy's filter_configs list

MINOR: flt-http-comp: Stop to explicitly loop on the proxy's filter_configs list

The "compression", "comp-req" and "comp-res" filter keyword parsers
scanned the filter_configs list of the proxy by hand to detect an
already declared compression filter configuration, matching on the
filter ids.

The parsers now use flt_find_conf(), the supported way to find a filter
configuration, which walks the right list depending on the filter mode of
the proxy. The "compression" duplicate check resolves the two possible
configurations with two successive lookups, one per direction.