Fixing 9162c3cb

Not fully solved, since the scannedDirs field can still grow to be large, but at least clearing files/dirsNotIncluded.
Also imposing a 5s timeout on the scan regardless of file count, and defining a user-customizable bound.

[JENKINS-25691] Might have prevented the need for: https://github.com/jenkinsci/maven-plugin/pull/33

ssh-slaves plugin can leverage this to try to obtain the state of the slave JVM via SSH.

Reference: ZD-23093

Reference: http://bugs.java.com/bugdatabase/view_bug.do?bug_id=8051847
Reference: ZD-23093

Reference: ZD-23093

Caused a failure in hudson.plugins.promoted_builds.conditions.ManualConditionTest.testManualPromotionProcessViaWebClient when run against 1.580.1.

Explode WAR fails when ./target does not yet exist (e.g., run from IDEA)

Allow Projects to take the Queue.Item's actions into consideration when determining their default authentication

- 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