RFR: JDK-8145442: Add an option to verify remembered sets for G1

[Poonam Bajaj Parhar]

Hello,

Please review these changes that add a diagnostic VM option 
'G1VerifyRSetsAfterEvacuationPause' that when turned on enables the 
verification of Rem Sets of heap regions.

Bug: JDK-8145442: Add an option to verify remembered sets for G1
Webrev: http://cr.openjdk.java.net/~poonam/8145442/webrev.00/
Testing: JPRT, GCBasher

Thanks,
Poonam