rec: keep cache warm for a list of names

[omoerbeek]

Short description

This is different from refresh-on-ttl-perc as it does not require a name/qtype to be queried to kick the refresh.

List is a keepwarm key in the recordcache section. Value is a list, for example:

recordcache:
  keepwarm:
    - qname: google.com
    - qname: microsoft.com

If you have lots of names in the list you'll see taskqueue-expired grow and taskqueue-size never hitting zero. In that case increase the new recursor.taskthreads setting.

Draft as tests and docs are missing.

Checklist

I have:

• read the CONTRIBUTING.md document
• read and accepted the Developer Certificate of Origin document, including the AI Policy, and added a "Signed-off-by" to my commits
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[coveralls]

Coverage Report for CI Build 27267339905

Warning

Build has drifted: This PR's base is out of sync with its target branch, so coverage data may include unrelated changes.
Quick fix: rebase this PR. Learn more →

Coverage remained the same at 71.116%

Details

• Coverage remained the same as the base build.
• Patch coverage: 41 uncovered changes across 4 files (246 of 287 lines covered, 85.71%).
• 83 coverage regressions across 16 files.

Uncovered Changes

File	Changed	Covered	%
pdns/recursordist/rec-main.cc	113	92	81.42%
pdns/recursordist/syncres.cc	96	81	84.38%
pdns/recursordist/rec-keepwarm.hh	26	23	88.46%
pdns/recursordist/rec-taskqueue.cc	9	7	77.78%
Total (11 files)	287	246	85.71%

Coverage Regressions

83 previously-covered lines in 16 files lost coverage.

Top 10 Files by Coverage Loss	Lines Losing Coverage	Coverage
pdns/recursordist/syncres.cc	23	81.1%
pdns/recursordist/rec-system-resolve.cc	19	49.32%
pdns/misc.cc	8	61.78%
pdns/recursordist/taskqueue.cc	5	64.1%
pdns/dnsdistdist/dnsdist-carbon.cc	4	59.56%
pdns/recursordist/recpacketcache.hh	3	91.14%
pdns/recursordist/taskqueue.hh	3	85.71%
pdns/recursordist/test-syncres_cc2.cc	3	79.35%
pdns/rcpgenerator.cc	2	89.45%
pdns/recursordist/lwres.cc	2	67.77%

---

Coverage Stats

Relevant Lines:	171043
Covered Lines:	133315
Line Coverage:	77.94%
Relevant Branches:	81710
Covered Branches:	46433
Branch Coverage:	56.83%
Branches in Coverage %:	Yes
Coverage Strength:	5882899.41 hits per line

---

💛 - Coveralls

[omoerbeek]

Rebased to fix conflicts