dht::sharder: add virtual destructor
dht::sharder: add virtual destructor

This is a class with virtual methods, it should have a virtual
destructor too.

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