Make Explain into Async Request

Currently when you go to the query detail page, the explain plan and other information is displayed. This requires several database queries to retrieve the information even if it's not needed. Since the info is hidden by default in an accordion widget, it should be possible to only get it when needed.

However the main issue to solve, is that the query sample has to be parsed by a plugin function first to even know if the explain is available, which means it's often hidden on the page. We'd need to know it was even retrievable at page creation time, so we know to display the appropriate section in the accordion widget.