First clean compile this week

I've finished roughing out the code for nested transactions. Now the fun begins.

Had to reorganize the cache classes a bit 'til I got it right. I had somehow forgotten that Block uses dependency and Pathnames and Topics needed transactional awareness.

Overall I'm happy with the code changes. Caches should work better now, besides doing more. And I eliminated the need to flush batches of Blocks--they're now only saved as needed to extend the file.

A good time to take some time off.

Bill