For robustness, pass on the null to Jelly rather than failing page rendering.

Jenkins.getComputers may be called frequently; do not perform an expensive locale-sensitive sort each time.

(Also relevant to JENKINS-22395 diagnostics.)

Start cron at zero seconds after the minute, and ensure the internal calendar is set to zero seconds.  Allows for more predictable behavior.

Instead, use a distance of 0.5x the width/height of the label.

This improves appearance in combination with JENKINS-22688 for
different font sizes.

pending was null as soon as task started running, so it was possible that another thread submits the task and have that executed while the first one was still running

This is a much simpler fix

- coding style was not matching the rest of Jenkins
- IOUtils.closeQuietly is a lot more concise way of achieving the same
  thing.
- AtomicFileWriter.abort() does close, so no need to close it one more
  time.

Bringing this to the parity with ParameterizedJobMixIn.doBuild()

Instead of returning JSON, it returns "201 Created" and responds with the location of the queue item, through which the caller can track how it passes through the queue and eventually gets built/cancelled.

The approach in pull request #750 that relies on
AbstractProject.getQueueItem() is inherently unreliable, because
item can be immediately built before the JSON is rendered, or it might
corresponds to other queued items if the build supports concurrent
builds, etc.

Massaging the pull request.

- Simplifying by reusing InterceptingExecutorService
- Not hiding the implementation class for better
  consistency with other ExecutorService impls in
  Jenkins.