compaction/lcs: add support for tombstone compaction
compaction/lcs: add support for tombstone compaction

LCS will choose its candidate by starting from highest level and
getting sstable which has highest droppable tombstone ratio.
Unlike STCS which needs to choose oldest sstable from biggest tier,
LCS can choose the one with highest d__t__r because sstables in
a given level don't overlap.
Sstable picked up for tombstone removal compaction won't be demoted
or promoted.

Signed-off-by: Raphael S. Carvalho <1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>

scylladb/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla