dnsdist: Handle congested DoQ streams

[rgacogne]

Short description

If the stream has no capacity left, Quiche will refuse to queue more data and return QUICHE_ERR_DONE. We then have to wait until the stream becomes writable again to retry sending our response.

Hopefully closes #13631. I managed to hit the reported issue and this PR fixes it for me, but I'm not 100% sure I am sending the same kind of traffic, so it might not fix all issues.

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 7224343063

• 44 of 108 (40.74%) changed or added relevant lines in 2 files are covered.
• 29 unchanged lines in 9 files lost coverage.
• Overall coverage decreased (-0.03%) to 57.671%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pdns/dnsdistdist/doq.cc	16	33	48.48%
pdns/dnsdistdist/doh3.cc	28	75	37.33%

Files with Coverage Reduction	New Missed Lines	%
pdns/dnsdistdist/doq.cc	1	72.71%
pdns/pollmplexer.cc	1	82.61%
pdns/signingpipe.cc	1	85.22%
ext/luawrapper/include/LuaContext.hpp	2	13.77%
modules/godbcbackend/sodbc.cc	2	70.8%
pdns/dnsdistdist/doh3.cc	2	61.44%
pdns/dnsdist-carbon.cc	6	59.34%
pdns/recursordist/test-syncres_cc1.cc	6	88.91%
pdns/packethandler.cc	8	72.84%

Totals
Change from base Build 7222412548:	-0.03%
Covered Lines:	107590
Relevant Lines:	155189

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💛 - Coveralls

[chbruyand]

LGTM!
Thanks for the fix