Should the atomicity be part of the adapter?

[mxstbr]

I don't know how many databases support atomic operations, but it might make sense to move the atomicity requirement to the adapters. That way if a database supports atomic operations the core doesn't do any unnecessary work.

This probably depends quite highly on #19, as this is hard to write and prove without a good test suite if the database does not have atomic operations. We could also provide a wrapper to make generic, non-atomic put operations atomic to make that step easier.