MINOR: cache: factor cache_extract_link_hints out of cache_extract_hints
Move the per-Link-value processing into a new cache_extract_link_hints()
helper. cache_extract_hints() continues to walk the cached row's HDR
blocks and now invokes the helper once per Link header it finds.
No behaviour change. This prepares the helper for reuse by an upcoming
hints-only storage path that extracts Link headers directly from the
live response HTX rather than from a cached row.