Snapshot soft ref policy before marking/copying

For users of `ReferenceProcessor`, set up the soft ref policy at the beginning of GC cycle, and use that policy across the whole GC cycle. IOW, the decision regarding clearing a soft ref will never give conflicting answers within a GC cycle. Therefore, the reconsideration of soft refs can be dropped (JDK-8051680).

Shenandoah and Z don't use `ReferenceProcessor`, but both set up the soft ref policy before marking.