Split the top level slab header hierarchy and the headers

Split the top level slab header hierarchy and the headers

The code that combines the top-level hierarchy (per-typepair) and
individual slab headers (per-version) saves a little bit of memory, but
makes the code convoluted, hard to read and hard to modify.  Change the
top level hierarchy to be of different type with individual slabheaders
"hanging" from the per-typepair dns_slabtop_t structure.

This change makes the future enhancements (changing the top level data
structure for faster lookups; coupling type + sig(type) into single
slabtop) much easier.