Be pseudo-atomic when expanding areas.
Be pseudo-atomic when expanding areas.

Areas are expanded in two operations. One to expand newspace, and one to
expand oldspace. These must be done while pseudo-atomic or another thread
can trigger a GC between the two expansions.

froggey/Mezzano,froggey/Mezzano,rvedam/Mezzano,varjagg/Mezzano,varjagg/Mezzano,AugustusHuang/Mezzano,AugustusHuang/Mezzano,rvedam/Mezzano