Revised stack extent and ForkJoinSlice grouping code
to avoid locking.
Refactored code so that both structs are in gc namespace while
StackExtent struct is no longer beneath StackExtents struct.
Bugfix: ForkJoinSlice destructor must clear it associated stack extent.
Remove unneeded include to reduce diff size from master.
Remove unnecessary public/private access exposure.
Put back assertion to reduce diff against master.
Remove unused gcRequestCount_ member.
Add note since this is the important bit in the code.
Removed now-unused next field.  Put private access back in.