write and dependency caches

Good progress on TKS today, though it still doesn't support nested transactions. BTree is also nearly complete. And Cache now supports writes as well as purgs when a transaction aborts.

But the two uses of cache are now quite distinct. Topics and Pathnames make use of dependencies, while Table and Block depend on purges and auto-writes. So I'm thinking there should be a base Cache, as well as a write Cache and a dependency Cache. Again, my thought is to keep things as understandable as possible. (And only the write cache needs to be aware of transactions.)

Overall, I still believe I will be finished with nested transactions this weekend, though I'm sure there will be a lot of debugging needed.

Bill