ixfrdist: add NOTIFY receive support

[Habbie]

Short description

part of #6994

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)
• checked that this code was merged to master

[coveralls]

Pull Request Test Coverage Report for Build 6576469821

• 101 of 146 (69.18%) changed or added relevant lines in 3 files are covered.
• 196 unchanged lines in 13 files lost coverage.
• Overall coverage decreased (-0.09%) to 57.406%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pdns/ixfrdist-stats.cc	8	10	80.0%
pdns/ixfrdist-stats.hh	0	3	0.0%
pdns/ixfrdist.cc	93	133	69.92%

Files with Coverage Reduction	New Missed Lines	%
pdns/misc.cc	2	62.39%
pdns/recursordist/rec-tcpout.cc	4	0.0%
pdns/recursordist/rec-zonetocache.cc	4	50.1%
pdns/ixfrdist.cc	5	58.68%
pdns/tcounters.hh	5	75.0%
pdns/mtasker.cc	6	67.02%
pdns/recursordist/rec_channel.cc	8	65.88%
pdns/recursordist/taskqueue.hh	9	42.86%
pdns/recursordist/recursor_cache.cc	10	76.37%
pdns/recursordist/rec-carbon.cc	12	0.0%

Totals
Change from base Build 6574953728:	-0.09%
Covered Lines:	105256
Relevant Lines:	152765

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💛 - Coveralls

[Habbie]

made clang-tidy happy

[rgacogne]

Looks good, a few remaining error messages, one suggestion.

[Habbie]

Thanks Rémi, I added two commits