Meta-issue: queue enhancements

A couple of queue enhancements have cropped up over time. This issue thread is trying to collect them.

• #872 (closed) (Add delays to typesetting)
• #873 (closed) (combine array of elements)
• better support for clearing the queue
  • this came up on the User Group but I can't find a link.
  • use case: content has been replaced dynamically by new content but some equations from the original content are still on the queue (e.g., epub3 reading systems have this problem)
  • IIUC, right now MathJax will fail gracefully if the original element disappears but it will still typeset the equations.
  • See also #933

What else?