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

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

The "cache" filter keyword parser and the implicit "cache-store"
declaration resolution both scanned the filter_configs list of the proxy
by hand to find a previously declared cache filter configuration. The
lookup filters were inlined in the loops, matching on the filter id and
the cache 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 implicit cache-store instance is
added directly with its class, without a name lookup through the class
registry, and the implicit configuration replaced by an explicit
declaration is removed with flt_conf_delete().