Gl next query

To get the discussion going about gl query objects.

(Previously named QueryTimeElapsed, I renamed it to QueryTimeSwapped here in order to make the swapping mechanism more explicit.)
Ah yes, end not stop, will fix. I used an exception here because I felt that CI_LOG_E was too weak since it was debug-mode only: most profiling will occur in release mode. Maybe you're right though, I'll review what happens when you read a query between begin() / end().