When table audit is enabled, Scylla creates the "audit" ks with
NetworkTopologyStrategy and RF=3. During node decommission, streaming can fail
for the audit ks with "zero replica after the removal" when all nodes from a DC
are removed, and so we have to ALTER audit ks to either zero the number of its
replicas, to allow for a clear decommission, or have them in the 2nd DC.
BTW. https://github.com/scylladb/scylladb/issues/27395 is the same change, but
in dtests repository.
Raft topology goes over all nodes in a 'left' state and triggers 'remove
node' notification in case id/ip mapping is available (meaning the node
left recently), but the problem is that, since the mapping is not removed
immediately, when multiple nodes are removed in succession a notification
for the same node can be sent several times. Fix that by sending
notification only if the node still exists in the peers table. It will
be removed by the first notification and following notification will not
be sent.
Closesscylladb/scylladb#27743
