rename last_offset/total_bytes in cof to help determine where
they are used (trying to see if this function can be moved to
the cof_index, since it is tracking that data too).
rename index_getattr_size to index_droppings_getattrsize so that
the function naming matches the other functions (index_dropping_*
functions go directly to the droppings -- don't assume the file
is open).