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.