RFR(S): 8202169: Reduce ctw_2 duration by parallelizing CtwRunner invocations

[Mikael Vidstedt]

Please review this change which parallelizes the CompileTheWorld/CtwRunner invocations to reduce the (wall clock) duration of running the ctw_2 jtreg test group.

bug: https://bugs.openjdk.java.net/browse/JDK-8202169 <https://bugs.openjdk.java.net/browse/JDK-8202169>
webrev: http://cr.openjdk.java.net/~mikael/webrevs/8202169/webrev.00/open/webrev/ <http://cr.openjdk.java.net/~mikael/webrevs/8202169/webrev.00/open/webrev/>

The CompileTheWorld class already has support for specifying the start and end class (index), so this change just enables passing the relevant arguments in to the CtwRunner wrapper class. For flexibility I made it possible both to specify the start/stop values both as absolute index, or as percentage values. This change only uses the latter format.

Cheers,
Mikael

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