Remove setting rdataset->ttl to 0. When it is set to 0, stale records
in "rndc dumpdb" output show 0 TTLs. Add rbtdb->serve_stale_ttl to various checks so that stale records are not purged from the cache when they've been stale for RBTDB_VIRTUAL (300) seconds. Increment ns_statscounter_usedstale when a stale answer is used. There's a question of whether overmem_purge should be purging ancient records, instead of stale ones. I've left it as purging stale records, since stale records could take up the majority of the cache.