Send the profiler output to the 'profileoutput' log group

And deprecate $wgProfileOnly in the same time.

This has the advantage of allowing profiler output to be separated
from the main debug log file; or even be completely disabled while
keeping the other debugging messages.

Also updated the checks in wfLogProfilingData() to detect the cases
where the output would not be sent anywhere to not execute the
last part of the method which would be useless otherwise.

Backward compatibility with installations having $wgProfileOnly
set to true is kept by moving the log file from $wgDebugLogFile
to $wgDebugLogGroups['profileoutput'] in Setup.php in that case.

Change-Id: I7b35195e527dfa7978b710126ed4599e75dab46b
Merge "Send the profiler output to the 'profileoutput' log group"