The set_scylla_local_param_as() wants to flush replica::database on all
shards. For that it uses smp::invoke_on_all() and qctx, but since the
method is now non-static one for system_keyspace it can enjoy usiing
container().invoke_on_all() and this->_db (on target shard)
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>