range_tombstone_list: Correctly implement difference()
range_tombstone_list: Correctly implement difference()

The difference method wasn't properly implemented. The version in this
patch correctly computes the difference and returns a range tombstone
list contains those range tombstones in "this" but absent from the
other, specified range tombstone list.

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

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