http2: tune control flow defaults #1592

Job	Run time
Build slim tarball	1m 31s
x86_64-darwin: with shared libraries	2h 53m 49s
aarch64-darwin: with shared libraries	2h 17m 32s
x86_64-linux: with shared libraries	2h 31m 45s
aarch64-linux: with shared libraries	2h 15m 25s
	10h 0m 2s

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