MINOR: filters: Mirror implicit filters as filter instances

MINOR: filters: Mirror implicit filters as filter instances

Add a helper to record an enabled implicit instance without replacing an
existing instance of the same class and id. Copy its raw arguments and
source location, and mark it as implicit so an explicit declaration can
replace it.

Create these instances alongside legacy filters for cache rules,
use-fcgi-app, and implicit compression/decompression. Cache and FCGI use the
cache/application name as the id; compression and decompression need none.
Allow the legacy FCGI configuration and a instance to refer to the same
application even though their configuration pointers differ.

Legacy filters are deliberately retained: reconciling the two models and
selecting which one is attached to streams remain unfinished.