ZGC: Check that all ZWorkers are initialized during startup

[Stefan Karlsson]

Hi all,

Please review this patch to check that all ZWorkers have been properly 
initialized during startup, and otherwise shutdown the VM.

http://cr.openjdk.java.net/~stefank/zgc/zPreInitializeZWorker/

Without this patch ZGC will hang if the diagnostic 
-XX:+InjectGCWorkerCreationFailure flag is used. It would also be 
problematic with, for example, ZMarkTerminate, if we didn't get all 
workers to start.

Thanks,
StefanK