In order to fix the race between split and repair, we must introduce
the ability to split an "offline" sstable, one that wasn't added
to any of the table's sstable set yet.
It's not safe to split a sstable after adding it to the set, because
a failure to split can result in unsplit data left in the set, causing
split to fail down the road, since the coordinator thinks this replica
has only split data in the set.
Refs #19378.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
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