io queue: make coordinator_of_shard const
io queue: make coordinator_of_shard const

It should have been like that from the start.

Signed-off-by: Glauber Costa <7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>

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