range_tombstone_list: Avoid violating set invariant
range_tombstone_list: Avoid violating set invariant

The code was inserting an entry with the same key as its successor,
and only later adjusting the key of the old entry. This is violating
set's invariant of unique keys, and insertion may cause rebalancing. I
don't know if this violation actually causes problems currently, but
it's safer not to.

Fix by first updating the existing entry and then inserting the new
one.

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