dnsdist: Fix Coverity warnings

[rgacogne]

Short description

One real issue that impact the TCP connection termination metrics, and some insignificant "performance" warnings in places where we do not care about performance.

The TCP metric issue was introduced in 448d66d so we will need to backport the fix to 1.8.x and perhaps 1.7.x.

Checklist

I have:

• read the CONTRIBUTING.md document
• 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]

Pull Request Test Coverage Report for Build 7250542226

Warning: This coverage report may be inaccurate.

We've detected an issue with your CI configuration that might affect the accuracy of this pull request's coverage report.
To ensure accuracy in future PRs, please see these guidelines.
A quick fix for this PR: rebase it; your next report should be accurate.

• 19 of 20 (95.0%) changed or added relevant lines in 3 files are covered.
• 1044 unchanged lines in 18 files lost coverage.
• Overall coverage increased (+0.003%) to 57.687%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pdns/dnsdist-lua.cc	0	1	0.0%

Files with Coverage Reduction	New Missed Lines	%
pdns/recursordist/settings/cxxsettings-generated.cc	2	91.55%
pdns/axfr-retriever.cc	3	65.53%
pdns/iputils.cc	4	45.31%
ext/yahttp/yahttp/router.hpp	5	35.71%
pdns/signingpipe.cc	5	85.22%
pdns/tcpiohandler.cc	5	66.38%
pdns/dnsdist-tcp.cc	9	75.72%
pdns/recursordist/rec-main.cc	12	32.39%
pdns/recursordist/test-syncres_cc1.cc	12	89.03%
pdns/ws-api.cc	13	87.31%

Totals
Change from base Build 7208535863:	0.003%
Covered Lines:	107607
Relevant Lines:	155189

---

💛 - Coveralls

[chbruyand]

LGTM!