zipping long blocks, queries, boolean

I've been giving some thought to snapshots and a threading model which supports queries while the database is being updated, but it will take some time to complete. A first-step is the support for loading objects as read-only and support for queries (as distinct from transactions which update the database).

I've also reinstated compression, but now only for serialized objects over a given limit--so you can tune for speed and still use compression. Boolean persistent objects are now also supported and a table can now have boolean values.

One bit of cleanup that I want to address before diving into snapshots is the problem with long running transactions. Dirty (updated) objects are currently kept in memory until the transaction completes and that is not a very nice restriction. Gotta fix it.