Add non-const overload of partition_start::partition_tombstone()
Add non-const overload of partition_start::partition_tombstone()

And make the const version return a const reference so that code
mutating the returned value won't compile if the partition_start object
is const.

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