MINOR: bwlim: Stop to explicitly loop on the proxy's filter_configs list

MINOR: bwlim: Stop to explicitly loop on the proxy's filter_configs list

The bandwidth limitation filter keyword parsers scanned the
filter_configs list of the proxy by hand to detect an already declared
"bwlim-in" or "bwlim-out" configuration, matching on the filter ids and
the limitation name.

The parsers now use flt_find_conf() with a match callback, the supported
way to find a filter configuration, which walks the right list depending
on the filter mode of the proxy. The duplicate check matches both filter
ids at once, with a single lookup.