Split memory management

Some attempts to address #1079 -- just for discussion at this point.
@michaelcahill, FWIW, it occurred to me that `WT_INCR_ADD` doesn't make a lot of sense to me `INCR` is kind of the same as `ADD`?   I'd suggest `WT_MEM_ADD`, `WT_SPACE_ADD`?   

Regardless, I didn't think about it until I was pretty far into review, so it's obviously not bugging me a lot.