dnsdist: Clean up the Lua objects before exiting

[rgacogne]

Short description

When code coverage is enabled, we try to call exit() instead of our usual use of _exit() to get more accurate coverage. It does however trigger a race condition between the destruction of Lua objects and the Lua context(s) they belong to, so we try to explicitly clean up the objects before calling exit().

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 7329839915

• 2 of 3 (66.67%) changed or added relevant lines in 1 file are covered.
• 46 unchanged lines in 11 files lost coverage.
• Overall coverage decreased (-0.02%) to 57.679%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pdns/dnsdist.cc	2	3	66.67%

Files with Coverage Reduction	New Missed Lines	%
pdns/dnsdist.cc	1	71.56%
pdns/query-local-address.cc	2	76.6%
pdns/rcpgenerator.cc	2	90.41%
pdns/ws-auth.cc	2	80.84%
modules/lmdbbackend/lmdbbackend.cc	3	72.35%
pdns/misc.hh	3	82.52%
pdns/iputils.cc	4	45.31%
pdns/signingpipe.cc	5	85.75%
pdns/dnsdist-carbon.cc	6	60.99%
pdns/recursordist/test-syncres_cc1.cc	6	88.99%

Totals
Change from base Build 7329684697:	-0.02%
Covered Lines:	107677
Relevant Lines:	155323

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💛 - Coveralls