dns: coalesce identical concurrent lookup() requests #5762

Job	Run time
Build slim tarball	2m 4s
aarch64-linux: with shared libraries	19m 31s
x86_64-darwin: with shared libraries	44m 15s
x86_64-linux: with shared libraries	23m 12s
aarch64-darwin: with shared libraries	18m 8s
	1h 47m 10s

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