Fix memory consumption in qp (Follow-up from "create QPDB zone database")

The following discussion from !8543 (merged) should be addressed:

• @pspacek started a discussion: (+1 comment)

  I gave it a quick look and sanity check - loaded net. TLD into this version (configured as secondary - named.conf).

  Here's a quick summary:

  metric	main	qpdb-heavy	4411-qpdb-lite
  zone load time	4:20	1:30	1:38
  memory RSS	6.2 GiB	12.8 GiB	12.8 GiB
  repeated bla.net. A query [QPS]	155 k	160 k	152 k
  repeated bla.net. A query [CPU utilization]	400 %	300 %	320 %
  random delegations [QPS]	150 k	150 k	148 k
  random delegations [CPU utilization]	470 %	370 %	410 %

  TL;DR smaller CPU usage while doubling amount of memory.

  Statistics channel output after just loading the zone (no queries yet):

  • main.json
  • heavy.json
  • lite.json