Less allocation

Michael, this branch allocates the WT_ROW, WT_COL and WT_REF page chunks at the same time as the WT_PAGE is allocated, and caches 100 WT_EXT structures to avoid allocation.   (I chose 100 for no particular reason, I don't see much reason to make it configurable, though.  I'm having a hard time imagining a workload that frees 1,000 chunks, then allocates 1,000 chunks, and repeats.  Thoughts?)