Another fix for the future node_monitors package extraction.

This time, a simple import removal.

- Also tidy up some of the mess logic in both #1866 and #1860

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]

- Also tidy up some holes in locking strategy that can cause builds to be scheduled on
  executors that are about to be interrupted or nodes in the process of being removed

The change adds some missing null checks
Signed-off-by: NOleg Nenashev <o.v.nenashev@gmail.com>

In 1.509.2, probably due to a bad Groovy script:
java.lang.ClassCastException: hudson.model.Hudson cannot be cast to hudson.model.Slave
	at hudson.slaves.SlaveComputer.getNode(SlaveComputer.java:176)
	at hudson.slaves.SlaveComputer.getNode(SlaveComputer.java:92)
	at jenkins.model.Jenkins$9.compare(Jenkins.java:1547)
	at jenkins.model.Jenkins$9.compare(Jenkins.java:1543)
	at java.util.Arrays.mergeSort(Arrays.java:1270)
	at java.util.Arrays.mergeSort(Arrays.java:1282)
	at java.util.Arrays.mergeSort(Arrays.java:1282)
	at java.util.Arrays.mergeSort(Arrays.java:1282)
	at java.util.Arrays.mergeSort(Arrays.java:1282)
	at java.util.Arrays.mergeSort(Arrays.java:1282)
	at java.util.Arrays.mergeSort(Arrays.java:1282)
	at java.util.Arrays.sort(Arrays.java:1210)
	at jenkins.model.Jenkins.getComputers(Jenkins.java:1543)

(04:07:53 PM) bbeck: In a plugin, given hudson.model.Node object is there an easy way to figure out how many busy/idle executors that node has?  There only seems to be a method to determine the number of executors, but not their states.
(04:24:02 PM) kohsuke: bbeck: see Computer
(04:25:51 PM) bbeck: kohsuke: thanks, just noticed the countIdle/countBusy functions there.

@sine -> @since

@lnk -> @link

@author: -> @author

Make embedded @ sign literal in one case where it is literal

Node monitoring should run in parallel to reduce the total round-trip
time in large instances.

... as per the discussion in the dev list

Related: https://groups.google.com/forum/?fromgroups#!topic/jenkinsci-dev/TMm-h6Hnb6Q

As Jesse pointed out, contextual information is often useful in authenticating the build, which means we need to take Queue.Item (initially AbstractBuild was the parameter, which provided the context, and I failed to accommodate that in transition to AbstractProject.)

To still allow Queue.Tasks to provide a meaningful value fallback to Queue.Task.getDefaultAuthentication() as opposed to hard code it to ACL.SYSTEM. This allow plugins like remote-terminal-access to supply a meaningful secure value without forcing a configuration change to the user.

java.lang.NullPointerException
	at hudson.tasks.CommandInterpreter.createScriptFile(CommandInterpreter.java:104)
	at hudson.tasks.CommandInterpreter.perform(CommandInterpreter.java:66)
	at hudson.tasks.CommandInterpreter.perform(CommandInterpreter.java:58)
	at com.cloudbees.hudson.plugins.modeling.impl.builder.BuilderImpl.perform(BuilderImpl.java:58)
	at hudson.plugins.m2extrasteps.M2ExtraStepsWrapper.executeBuildSteps(M2ExtraStepsWrapper.java:166)
	at hudson.plugins.m2extrasteps.M2ExtraStepsWrapper.access$200(M2ExtraStepsWrapper.java:43)
	at hudson.plugins.m2extrasteps.M2ExtraStepsWrapper$1.tearDown(M2ExtraStepsWrapper.java:137)
	at hudson.maven.MavenModuleSetBuild$RunnerImpl.doRun(MavenModuleSetBuild.java:817)
	at hudson.model.AbstractBuild$AbstractRunner.run(AbstractBuild.java:490)
	at hudson.model.Run.run(Run.java:1442)
	at hudson.maven.MavenModuleSetBuild.run(MavenModuleSetBuild.java:477)
	at hudson.model.ResourceController.execute(ResourceController.java:88)
	at hudson.model.Executor.run(Executor.java:239)

Fied a couple of places where Task.getAssignedLabel() was invoked directly without checking LabelAssignmentAction

Fix code that accesses functionality though package protected interfaces;
- access through AbstractCIBase
- make FullDuplexHttpChannel and OverallLoadStatistics public

Test case utility function for adjusting the quiet period variable
Signed-off-by: NNigel Magnay <nigel.magnay@gmail.com>

Just import changes, from hudson.model.Jenkins -> jenkins.model.Jenkins,
and Javadoc comments.
Signed-off-by: NNigel Magnay <nigel.magnay@gmail.com>

Signed-off-by: NNigel Magnay <nigel.magnay@gmail.com>