Make a separate NodeReader role for the benefit of the verifier code
Add 16 bytes of NULLs as a separator between the search tree & data section

Empty records are now indicated by a value == number of nodes

Taken together, these changes make it possible for a verifier to reliably
distinguish between the search tree and data section.

Previously, it would have been possible for the search tree to contain an
arbitrarily long string of NULLs (X empty records), so there was no unique
value that could distinguish the search tree from the data sction.