http: reject addTrailers after finish #5639

Job	Run time
Build slim tarball	2m 8s
x86_64-linux: with shared libraries	24m 44s
x86_64-darwin: with shared libraries	27m 17s
aarch64-linux: with shared libraries	20m 37s
aarch64-darwin: with shared libraries	17m 46s
	1h 32m 32s

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