MEDIUM: cache: early hints-aware eviction code

MEDIUM: cache: early hints-aware eviction code

We now use the make_room callback from shctx to customize the eviction
procedure. We attempt to balance eviction between hints and full-blown
entries by capping the number of blocks used by hints entries to 25% of
the size of the cache. Upon eviction of a full entry, we first try to
strip it to a hints-only entry if possible.

A Link header's value (which may carry multiple comma-separated links)
is limited to 1024 bytes for hint extraction. Longer values are skipped
entirely and contribute no hint records.