RFR: 8152113: Remove _last_ditch_collection GC-cause and avoid expanding heap on Metaspace OOM

[Stefan Johansson]

Hi,

Please review this change for RFE:
https://bugs.openjdk.java.net/browse/JDK-8152113

Webrev:
http://cr.openjdk.java.net/~sjohanss/8152113/hotspot.00/

Summary:
Prior to this patch the GC-cause _last_ditch_collection has been used 
for 2 types of full-collections that clears soft references:
1. Last resort when out of metaspace memory
2. WhiteBox initiated full GC

These have not much in common and the GC-cause is strangely named when 
looking at its meaning. This cause also comes with an unwanted 
side-effect, it will expand the heap if possible and this is strange 
since it is not caused by a failed heap-allocation. This change removes 
the _last_ditch_collection cause and now we instead use two separate 
causes, _wb_full_gc for the WhiteBox-case and 
_metadata_GC_clear_soft_refs for the out of metaspace case. The idea is 
that these will work similar to _last_ditch_collection in every way 
except when it comes to growing the heap.

Testing:
Run several tonga test-lists in RBT with G1, CMS and Parallel without 
seeing any new issues.

Thanks,
Stefan