Improve totaltime calculation

In phpbb, we are calculating the total processing time of a page, but there are currently two issues, or rather code that can be improved:

• $starttime is currently set in startup.php, and used as global in other methods.
  This appears unneeded: where we need the starttime, we could as well use a variable set by PHP:

  request->server('REQUEST_TIME_FLOAT', 0.0)

• For starttime, some code uses the string return version of microtime(), to then split the result and sum both parts. This should be replaced with microtime(true). Example from driver.php

$mtime = explode(' ', microtime())