RFR: Small change to logging to make -Xlog:gc behave better.

[Christine Flood]

I changed the behavior slightly so that -Xlog:gc now 
reports:

   init mark pause times
   final mark pause times
   concurrent marking times
   concurrent evacuation times
   and when concurrent gc is canceled. 


http://cr.openjdk.java.net/~chf/BetterLogging/webrev.00/

Christine