Remove internal use of deprecated $wgRequestTime

Remove internal use of deprecated $wgRequestTime

* Use $_SERVER['REQUEST_TIME_FLOAT'] unconditionally in WebRequest.php
  and libs/Timing.php. WebStart.php was doing this already without issue.
  The key existst since PHP 5.4, for both Web and CLI (we require 5.5).

* In wfDebug() and wfReportTime(), use $_SERVER['REQUEST_TIME_FLOAT'] instead.

* In ApiFormatBase and MWDebug, use WebRequest::getElapsedTime() instead.

* In Maintenance.php, remove setting of $wgRequestTime.

* In rebuildFileCache.php, update mocking to $_SERVER['REQUEST_TIME_FLOAT']
  so that we avoid re-introducing bug T24852.

Change-Id: I1b647da2862f815029caa533b592ec8a05b33806