Praefect: replication processing should omit unhealthy nodes

Replication should consider the state of the target nodes and omit dequeueing and processing of replication events for unhealthy nodes. Otherwise replication events will be execute maximum attempts without success and marked as dead. So after healing the node will be outdated until the new replication will be executed.
This change will allow to pick up not executed events only after node will be healthy, so this may prevent repository from being outdated after it becomes healthy again.

Closes: #2922 (closed)