Remove most uses of WT_STORE_SIZE, it was primarily used to convert
size_t types to uint32_t types, during assignment to the WT_ITEM.size
field.
Replace WT_PTRDIFF32 with WT_PTRDIFF where we're storing into a
WT_ITEM.size field.
Don't leave random big files lying around.
Don't recreate the directory when exiting successfully.