compaction strategies know how to pick files that are most likely to
satisfy tombstone purge conditions (i.e. not shadow data in uncompacting
files).
This logic can be bypassed if tombstone GC was disabled by user,
as it's a waste of effort to proceed with it until re-enabled.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>