Finalmask: Refactor header conns to avoid multiple-copy; Add `randRange` to "header-custom" (TCP & UDP) #6930

Job	Run time
check-assets	6s
build (freebsd, amd64)	1m 8s
build (android, arm64)	1m 28s
build (darwin, amd64)	1m 11s
build (openbsd, amd64)	1m 7s
build (linux, amd64)	1m 13s
build (linux, arm, 7)	1m 13s
build (linux, arm, 6)	1m 1s
build (freebsd, 386)	1m 10s
build (linux, ppc64)	1m 6s
build (linux, loong64)	1m 9s
build (linux, mips64)	1m 6s
build (linux, mips64le)	1m 10s
build (openbsd, 386)	1m 12s
build (linux, riscv64)	1m 8s
build (linux, arm, 5)	1m 11s
build (linux, ppc64le)	1m 7s
build (linux, 386)	1m 6s
build (darwin, arm64)	1m 12s
build (freebsd, arm, 7)	32s
build (freebsd, arm64)	1m 10s
build (openbsd, arm64)	1m 5s
build (linux, mips)	1m 30s
build (android, amd64, android-amd64)	1m 33s
build (linux, mipsle)	1m 32s
build (linux, s390x)	1m 12s
build (linux, arm64)	1m 3s
build (windows, 386)	1m 4s
build (openbsd, arm, 7)	1m 7s
build (windows, arm64)	1m 4s
build (windows, amd64)	1m 3s
	34m 59s

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