http: reject addTrailers after finish #5737

Job	Run time
Build slim tarball	3m 11s
x86_64-linux: with shared libraries	24m 27s
aarch64-linux: with shared libraries	20m 46s
aarch64-darwin: with shared libraries	20m 39s
x86_64-darwin: with shared libraries	29m 9s
	1h 38m 12s

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