src: cache context lookup in vectored io loops #2375

Job	Run time
Build slim tarball	1m 28s
x86_64-darwin: with shared libraries	47m 20s
x86_64-linux: with shared libraries	32m 18s
aarch64-linux: with shared libraries	25m 59s
aarch64-darwin: with shared libraries	30m 46s
	2h 17m 51s

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