FingerprintCleanupThread ought to write messages to its saved log file.
Report a warning to build log when the selected JDK cannot be found.
[FIXED JENKINS-17667] Fixed race condition when running tool installers on many slaves at once.
Correcting change made in #1176, which introduced an NPE, to restore original logic merely wrapped in a synchronized block.
Reproduced NPE in new functional test (original bug probably very hard to reproduce).