Warn in Queue.init if items scheduled during startup will be clobbered

[jglick]

I tracked down a subtle flaw in ExecutorPickle. This warning would have made the root cause obvious long ago.

Not adding a warning in load because that is called from Jenkins.reload which saves the Queue first. Still has a race condition but that is another matter.

Proposed changelog entries

• N/A

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[basil]

This PR is now ready for merge. We will merge it after approximately 24 hours if there is no negative feedback. Please see the merge process documentation for more information about the merge process. Thanks!