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

https://tracker.phpbb.com/browse/PHPBB3-12687

---------

In phpBB 3.0.x the DEBUG constant had a very little impact: track the sql queries and display the load time. And many user (maybe something like 70%) like to display the load time of the page to the user and so they set the DEBUG constant to true.

In 3.1.x the DEBUG constant add many other things:
- twig debug
- dump or not url_matcher
- search or not the templates events on each page
- ...

And so the impact of the DEBUG constant on the performances isn't negligible at all.

But the users still want to display the load time, and if we tell them to set the DEBUG constant to true they all will complain because their board will take something like 0.7s on each page (and for that the support team will want kill us xD)

So we need to managed the displaying of the load time information independently of the DEBUG constant.

---------
Will be revert in 3.2 by ticket/12620 (#2521)

PHPBB3-12687
Check for $starttime being defined. I guess we could also just use $GLOBALS['starttime'] instead of importing it via the global statement.