CowDb has too many kinds of tables

There is a needless excess of table and hash table types in C0wDb. This makes it harder to learn, and that's a no no. Really, we need only two:

1. Large tables (b-trees), which can hold PObjects as well as references to logical blocks, and
2. Large extensible hash tables.

This will bring some much needed clarity to CowDb. And that is very important. But right now, I'm working on a major hole in the reference counting logic. :-(