RFR(S): 8232232: G1RemSetSummary::_rs_threads_vtimes is not initialized to zero

[Man Cao]

Hi all,

Can I have reviews for this fix for logging messages of "Concurrent
refinement threads times (s)", and code cleanup?

Webrev: https://cr.openjdk.java.net/~manc/8232232/webrev.00/
Bug: https://bugs.openjdk.java.net/browse/JDK-8232232

-Man