http: reject addTrailers after finish #5642

Job	Run time
Build slim tarball	1m 58s
x86_64-darwin: with shared libraries	40m 49s
aarch64-darwin: with shared libraries	19m 32s
aarch64-linux: with shared libraries	18m 52s
x86_64-linux: with shared libraries	20m 30s
	1h 41m 41s

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