MEDIUM: pattern: make pat_ref_prune() rely on pat_ref_purge_older()

MEDIUM: pattern: make pat_ref_prune() rely on pat_ref_purge_older()

When purging all of a reference, it's much more efficient to scan the
reference patterns from the reference head and delete all derivative
patterns than to scan the expressions. The only thing is that we need
to proceed both for the current and next generations, in case there is
a huge gap between the two. With this, purging 20M IP addresses in small
batches of 100 takes roughly 3 seconds.