sstables: wrap atomic deletion code in a class
sstables: wrap atomic deletion code in a class

This makes it easier to abstract and unit-test.

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