Cap glue records cached from a referral

Cap glue records cached from a referral

The resolver populated the delegation database with every NS RR and
every glue address from a referral, with no aggregate bound.  Resolution
only ever uses the first max-delegation-servers NS owners and a handful
of addresses per NS, so anything beyond that is dead memory.

Stop the NS loop in cache_delegns() at view->max_delegation_servers and
cap each glue rdataset at DELEG_MAX_GLUES_PER_NS (20) addresses, so each
NS owner contributes at most 20 A and 20 AAAA glues.