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

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

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

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. As a side effect, the message of the "decomp-res" duplicate
error now names the right filter.