counters: implement decrement operators for shard_iterator
counters: implement decrement operators for shard_iterator

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