RFR: Defer cleaning of system dictionary and friends to parallel cleaning phase

[Roman Kennke]

Found this during traversal GC work: when cleaning the system dictionary 
and friends, we do clean it in the first pass, *single threaded* and 
then do the cleaning stuff again, but multi-threaded. We shall defer 
cleaning to the parallel phase to begin with. That's what G1 does too.

http://cr.openjdk.java.net/~rkennke/defer_cleaning/webrev.00/

Ok?

Roman