Add loadgen "stop" mode #6452

Job	Run time
fmt	16s
build (gcc, current)	37m 15s
rust-check-git-rev-deps	27s
build (gcc, current, --check-test-tx-meta)	20m 18s
cargo-deny (advisories)	1m 8s
build (gcc, next)	38m 17s
cargo-deny (bans licenses sources)	49s
build (gcc, next, --check-test-tx-meta)	21m 59s
build (clang, current)	31m 9s
build (clang, current, --check-test-tx-meta)	18m 56s
build (clang, next)	32m 50s
build (clang, next, --check-test-tx-meta)	20m 20s
complete	1s
	3h 43m 45s

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