[JENKINS-31055] Note theoretical race condtion

Seems rather unlikely, but none the less it is a possibility. Not sure how we should handle this though:

* We do not want to hold the Queue lock while performing I/O as that will grind everything to a halt
* We do not want a big fat lock on all nodes
* If we have a per-node lock then the save everything method may have to grab them all

In short, it gets ugly real fast.

For now, I believe that the risk is very low as releasing a lock does not intrinsically deschedule a currently running thread so the likely course of events is that the thread will release the lock and at least start the save before a competing thread could acquire the lock update remove the node release the lock and delete the node's directory

core/src/main/java/jenkins/model/Nodes.java

@@ -138,6 +138,7 @@ public class Nodes implements Saveable {
                     jenkins.trimLabels();
                 }
             });
+            // TODO there is a theoretical race whereby the node instance is updated/removed after lock release
             persistNode(node);
         }
     }
@@ -191,6 +192,7 @@ public class Nodes implements Saveable {
             exists = false;
         }
         if (exists) {
+            // TODO there is a theoretical race whereby the node instance is updated/removed after lock release
             persistNode(node);
             return true;
         }