chg: usr: Limit the number of glue records cached from a referral

When a delegation response contained many glue addresses per listed
nameserver, all of them were cached without a per-nameserver bound,
inflating resolver cache memory beyond what resolution could ever use.
The cache now keeps at most 20 IPv4 and 20 IPv6 glue addresses per
nameserver from a delegation.

Closes #5701

Merge branch '5701-limit-the-number-of-GLUE-records-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11972