RFR: 8059466: Force young GC to initiate marking cycle when stat update is requested

[Stefan Johansson]

Hi,

Please review this change to allow status updates to initiate a marking 
cycle:
https://bugs.openjdk.java.net/browse/JDK-8059466

Webrev:
http://cr.openjdk.java.net/~sjohanss/8059466/webrev.00/

Summary:
The "update_allocation_context_stats" GC cause has been split up to two 
causes, one for FullGC and one for young. The young one can be used to 
start a marking cycle.

Thanks,
Stefan