BIND 9.17.20 leaks memory when used as a recursive resolver

BIND 9.17.20 release testing revealed that memory use grows significantly over time when named is used as a resolver.

This was already suspected in #2953, but the problem was not as apparent back then.

The tricky part about finding the root cause of this problem is that it does not manifest itself all the time. For example, consider the following 3 jobs, all of which were ran over the weekend 2 weeks ago for the exact same code revision (the results quoted come from FreeBSD jobs):

• https://gitlab.isc.org/isc-projects/bind9/-/jobs/2065864

  OK (click to expand)

  2021-10-23:10:31:17 INFO: analysis for directory /var/tmp/gitlab_runner/builds/e-TSUMFs/0/isc-projects/bind9/output/ns3
  Total of 121 measurements
  Mid point of run:
     Minimum: 303,776 KiB
     Maximum: 333,344 KiB
     Average: 314,357 KiB
  End of run:
     Minimum: 312,740 KiB
     Maximum: 341,716 KiB
     Average: 325,141 KiB

• https://gitlab.isc.org/isc-projects/bind9/-/jobs/2066591

  OK (click to expand)

  2021-10-24:10:31:39 INFO: analysis for directory /var/tmp/gitlab_runner/builds/e-TSUMFs/0/isc-projects/bind9/output/ns3
  Total of 121 measurements
  Mid point of run:
     Minimum: 291,820 KiB
     Maximum: 319,860 KiB
     Average: 305,058 KiB
  End of run:
     Minimum: 298,416 KiB
     Maximum: 336,140 KiB
     Average: 315,823 KiB

• https://gitlab.isc.org/isc-projects/bind9/-/jobs/2067629

  ERROR (click to expand)

  2021-10-25:10:31:52 INFO: analysis for directory /var/tmp/gitlab_runner/builds/e-TSUMFs/0/isc-projects/bind9/output/ns3
  Total of 121 measurements
  Mid point of run:
     Minimum: 376,972 KiB
     Maximum: 547,596 KiB
     Average: 452,970 KiB
  End of run:
     Minimum: 595,488 KiB
     Maximum: 747,184 KiB
     Average: 676,506 KiB

We need to get to the bottom of this before BIND 9.18.0 is released.