dnsdist: Add warnings about large values passed to setMaxTCPClientThreads

[rgacogne]

Short description

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 10142819164

Details

• 3 of 5 (60.0%) changed or added relevant lines in 1 file are covered.
• 46 unchanged lines in 12 files lost coverage.
• Overall coverage decreased (-0.01%) to 64.634%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pdns/dnsdistdist/dnsdist.cc	3	5	60.0%

Files with Coverage Reduction	New Missed Lines	%
pdns/ueberbackend.cc	1	81.85%
pdns/recursordist/syncres.cc	1	79.42%
pdns/recursordist/aggressive_nsec.cc	2	66.39%
ext/json11/json11.cpp	2	62.72%
pdns/rcpgenerator.cc	2	89.86%
pdns/misc.cc	2	63.26%
pdns/dnsdistdist/dnsdist.cc	2	68.54%
pdns/recursordist/test-syncres_cc2.cc	3	88.91%
pdns/opensslsigners.cc	3	61.34%
pdns/recursordist/rec-tcpout.cc	5	50.79%

Totals
Change from base Build 10139890791:	-0.01%
Covered Lines:	124575
Relevant Lines:	162046

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💛 - Coveralls