Before this patch, we didn't send the `barrier_and_drain` command
to a decommissioning node that could still be coordinating
requests. It could happen that a decommissioning node sent
a request with an old topology version after normal nodes received
the new fence version. Then, the request would fail on replicas
with the stale topology exception.
We fix this problem by modifying `exec_global_command`. From now
on, it sends `barrier_and_drain` to a decommissioning node, which
can also be in the `left_token_ring` state.
9aebd6dd96