storage_service: Move cross-shard hop up the stack
storage_service: Move cross-shard hop up the stack

The storage_service::node_ops_cmd_handler runs inside a huge
invoke_on(0, ...) lambda. Make it be called on shard-0. This
is the preparation for next two patches.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

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