Tablet split can call set_split_mode() between the point where
truncate_table_on_all_shards() disables compaction on all existing
compaction groups and the point where discard_sstables() checks that
compaction is disabled. The new split-ready compaction groups created
by set_split_mode() won't have compaction disabled, causing
discard_sstables() to fire on_internal_error.
Fix by preventing set_split_mode() from creating new compaction groups
when compaction is disabled on the main group. If truncation has
already disabled compaction, split will simply report not-ready rather
than creating groups which have compaction enabled.
This is safe because split will be retried once truncation completes
and re-enables compaction.
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