RFR(S): 8011343: Add new flag for verifying the heap during startup

[John Cuthbertson]

Hi Everyone,

Can I have a couple of reviews for this change please? The webrev can be 
found at: http://cr.openjdk.java.net/~johnc/8011343/webrev.0/

Summary:
During the code review for 8010463 it was suggested that the 
verification that is performed during VM startup should be done under 
control of a new flag rather than piggy-backing off the VerifyBeforeGC 
flag. In response to another comment I suggested that the verification 
should be done using a VM Operation so as to maximize the verification 
(certain parts of the G1 heap are skipped if the verification is not 
performed at a safepoint).

These changes address these comments.

Testing:
GC test suite with the new flag
jprt with the new flag (this configuration saw some failures that are 
caused by the issue described in 8010943)
jtreg with the changed regression test

Note: I think the warning(s) emitted from arguments.cpp when 
FLSVerifyAllHeapReferences is enabled can be removed (now that 
verification is performed at a safepoint) but I am loathe to do so. 
Testing this configuration resulted in some assertion failures (even 
with a new workspace) that I'll be submitting another CR for.

Thanks,

JohnC