Off-strategy suffers with a 100% space overhead, as it adopted
a sort of all or nothing approach. Meaning all input sstables,
living in maintenance set, are kept alive until they're all
reshaped according to the strategy criteria.
Input sstables in off-strategy are very likely to be mostly disjoint,
so it can greatly benefit from incremental compaction.
The incremental compaction approach is not only good for
decreasing disk usage, but also memory usage (as metadata of
input and output live in memory), and file desc count, which
takes memory away from OS.
Turns out that this approach also greatly simplifies the
off-strategy impl in compaction manager, as it no longer have
to maintain new unused sstables and mark them for
deletion on failure, and also unlink intermediary sstables
used between reshape rounds.
Fixes https://github.com/scylladb/scylladb/issues/14992.
Closesscylladb/scylladb#15400
* github.com:scylladb/scylladb:
test: Verify that off-strategy can do incremental compaction
compaction: Clear pending_replacement list when tombstone GC is disabled
compaction: Enable incremental compaction on off-strategy
compaction: Extend reshape type to allow for incremental compaction
compaction: Move reshape_compaction in the source
compaction: Enable incremental compaction only if replacer callback is engaged
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