Merge pull request #2570 from Nicofuma/ticket/12687

[ticket/12687] Add a constant to display the load time without DEBUG

* Nicofuma/ticket/12687:
  [ticket/12687] Fix the namespace of $request
  [ticket/12687] Update doc block
  [ticket/12687] Use microtime(true)
  [ticket/12687] Remove globals
  [ticket/12687] Return a string and move the displaying of the sql report
  [ticket/12687] Rename DISPLAY_LOAD_TIME to PHPBB_DISPLAY_LOAD_TIME
  [ticket/12687] Display the explain link only when DEBUG is set
  [ticket/12687] Add a constant to display the load time without DEBUG