Raft replication doesn't guarantee that all replicas see
identical Raft state at all times, it only guarantees the
same order of events on all replicas.
When comparing raft state with gossip state on a node, first
issue a read barrier to ensure the node has the latest raft state.
To issue a read barrier it is sufficient to alter a non-existing
state: in order to validate the DDL the node needs to sync with the
leader and fetch its latest group0 state.
Fixes#13518 (flaky topology test).
Closes#13756
e7c9ca560b