[FIXED JENKINS-27565] Refactor the Queue and Nodes to use a consistent locking strategy

The test system I set up to verify resolution of customer(s)' issues driving this change, required
additional changes in order to fully resolve the issues at hand. As a result I am bundling these
changes:

- Moves nodes to being store in separate config files outside of the main config file (improves performance) [FIXED JENKINS-27562]
- Makes the Jenkins is loading screen not block on the extensions loading lock [FIXED JENKINS-27563]
- Removes race condition rendering the list of executors [FIXED JENKINS-27564] [FIXED JENKINS-15355]
- Tidy up the locks that were causing deadlocks with the once retention strategy in durable tasks [FIXED JENKINS-27476]
- Remove any requirement from Jenkins Core to lock on the Queue when rendering the Jenkins UI [FIXED-JENKINS-27566]

core/src/main/java/hudson/Functions.java

@@ -28,6 +28,7 @@ package hudson;
 import hudson.cli.CLICommand;
 import hudson.console.ConsoleAnnotationDescriptor;
 import hudson.console.ConsoleAnnotatorFactory;
+import hudson.init.InitMilestone;
 import hudson.model.AbstractProject;
 import hudson.model.Action;
 import hudson.model.Describable;
@@ -201,7 +202,12 @@ public class Functions {
     public static String rfc822Date(Calendar cal) {
         return Util.RFC822_DATETIME_FORMATTER.format(cal.getTime());
     }
-    
+
+    public static boolean isExtensionsAvailable() {
+        final Jenkins jenkins = Jenkins.getInstance();
+        return jenkins == null ? false : jenkins.getInitLevel().compareTo(InitMilestone.EXTENSIONS_AUGMENTED) >= 0;
+    }
+
     public static void initPageVariables(JellyContext context) {
         StaplerRequest currentRequest = Stapler.getCurrentRequest();
         String rootURL = currentRequest.getContextPath();