'searchRate' trigger: belowNodeOp=DELETENODE can result in loss of leader

While working on SOLR-13140 I discovered that configuring a very high belowNodeRate in SearchRateTriggerIntegrationTest.testDeleteNode can cause all nodes – even the node hosting the shard leader – to be the target of DELETENODE ops.

this indicates at least one serious bug in the code (we should never allow the leader to be deleted), but also raises other questions about situations not adequately tested:

• even if the code isn't particularly protecting the leader, why isn't minReplicas protecting at least one replica?
• what would happen if multiple replicas co-existed on the same node? would if the leader was one of the replicas that existed on the same node as another replica?
• what would happen if there were additional collections in the cluster that had replicas on these nodes that had low search rate for this target collection? would they protect the nodes from being the target of DELETENODE ops.