range_tombstone_list: Make tombstone merging commutative
range_tombstone_list: Make tombstone merging commutative

Example of non-commutative case:

 a = [1, 5]@t2
 b = {[2, 3]@t1, [4, 5]@t1}

 a + b = [1, 5]@t2
 b + a = [1, 4)@t2, [4, 5]@t2

After this patch, both will yield [1, 5]@t2.

The patch also changes the logic to handle overlaps of tombstones with
equal timestamps to be handled symmetrically. They are now merged
instead of split on either of the boundary.

Refs #2158.

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