That tombstone_histogram is used to determine droppable data ratio
for a sstable, and unlike C*, we were only updating it for
tombstones. We need to update it with expiration time of cells too,
if any. Creation time (expiration - ttl) cannot be used because if
ttl > gc_grace_seconds, the resulting sstable could be considered
worth dropping by tomstone compaction before any data is actually
expired.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
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