compaction_strategy: use duration type for default tombstone compaction interval
compaction_strategy: use duration type for default tombstone compaction interval

Signed-off-by: Raphael S. Carvalho <1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>
Message-Id: <20170630041838.20604-1-1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>

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