core/sharded: Make async_sharded_service dtor virtual
core/sharded: Make async_sharded_service dtor virtual

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>
Message-Id: <20180326191008.83816-1-a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

scylladb/seastar,cloudius-systems/seastar,avikivity/seastar,cloudius-systems/seastar,avikivity/seastar,cloudius-systems/seastar,scylladb/seastar,syuu1228/seastar,scylladb/seastar,avikivity/seastar,dreamsxin/seastar,syuu1228/seastar,syuu1228/seastar,dreamsxin/seastar,dreamsxin/seastar