http2: optimize neverIndex lookup with SafeSet #1978

Job	Run time
Build slim tarball	1m 31s
x86_64-linux: with shared libraries	29m 10s
aarch64-linux: with shared libraries	25m 1s
aarch64-darwin: with shared libraries	22m 49s
x86_64-darwin: with shared libraries	45m 58s
	2h 4m 29s

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