Exceeded time limit waiting for 'too many DNS UPDATEs queued' in ns1/named.run on Windows

This happens way to often on Windows (3393969 & 3394153 just today).

I:nsupdate:check that update is rejected if quota is exceeded (62)
I:nsupdate:exceeded time limit waiting for 'too many DNS UPDATEs queued' in ns1/named.run
I:nsupdate:failed

In #3846 (closed), we bumped the number of parallel queries from 10 to 20, but I don't think bumping it to even more is the way to go.

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