JIRA was down when I've created the issue

NodeProvisioner#getPendingLaunches() and NodeProvisioner#update() may get deadlocked if somebody retrieves a list of pending launches in the build scheduler. It
also applies to all other methods guarded by provisioningLock, because NodeProvisioner#update() locks the queue after locking provisioningLock.

Pull requests:

• https://github.com/jenkinsci/jenkins/pull/1777 (queue-locking implementation, declined)
• https://github.com/jenkinsci/jenkins/pull/1785 (lock-free implementation, accepted)