LCS reshape is compacting all levels if a single one breaks
disjointness. That's unnecessary work because rewriting that single
level is enough to restore disjointness. If multiple levels break
disjointness, they'll each be reshaped in its own iteration, so
reducing operation time for each step and disk space requirement,
as input files can be released incrementally.
Incremental compaction is not applied to reshape yet, so we need to
avoid "major compaction", to avoid the space overhead.
But space overhead is not the only problem, the inefficiency, when
deciding what to reshape when overlapping is detected, motivated
this patch.
Fixes#12495.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
Closes#12496
f2f839b9cc