dns: coalesce identical concurrent lookup() requests #5074

Job	Run time
Build slim tarball	1m 52s
x86_64-linux: with shared libraries	22m 10s
aarch64-darwin: with shared libraries	18m 23s
aarch64-linux: with shared libraries	18m 21s
x86_64-darwin: with shared libraries	40m 58s
	1h 41m 44s

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