The sl-controller is stopped in three steps. The first (and instantly the second) is unsubscribing from lifecycle notification and draining. The third is stop itself. First two steps are "out of order" as compared to the desired start-stop sequence of any service, this patch fixes these steps.
After this PR the drain_on_shutdown() (the call that drains the node upon stop) finally becomes clean and tidy and is no longer accompanied by ad-hoc fellow drains/stops/aborts/whatever.
refs: #2737Closesscylladb/scylladb#18731
* github.com:scylladb/scylladb:
sl_controller: Remove drain() method
sl_controller: Move abort kicking into do_abort()
main,sl_controller: Subscribe for early abort
main: Unsubscribe sl controller next to subscribing
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