dns_adb: old buckets are not cleaned up

This is two-fold:

1. isc_ht needs a shrinking process (which is similar to growing, but little bit more complicated, because adding while shrinking could overcommit the table a lot)
2. dns_adb: needs to garbage collect the old buckets and remove the empty buckets from isc_ht, so the isc_ht can actually shrink