It was reported in #2779 that the primary was constantly logging messages like `Synchronous replication key updated by someone else`.
It happened after Patroni was stuck due to resource starvation. Key updates are performed using create_revision/mod_revision field, which value is taken from the internal cached. Hence, it is a clear symptom of stale cache.
Similar issues in K8s implementation were addressed by invalidating the cache and restarting watcher connections every time when update failed due to resource_version mismatch, so we do the same for Etcd3.
