Switch MarkRoots to use mCurrNode rather than a local.

This is in preparation for making marking incremental.
Pass in budget to MarkRoots
incrementalize MarkRoots
Prepare RootGarbage for incrementalization.

Move the RootGarbage prelude into the end of ScanRoots

Convert local vars in RootGarbage to fields on nsCycleCollector
Incrementalize RootGarbage
Prepare to make UnlinkGarbage incremental

Turn the local loop var into an nsCC field
Make UnlinkGarbage incremental
prepare for incrementalizing UnrootGarbage
Incrementalize UnrootGarbage
fix up some checkpoint logging