DBDB (data-store) chapter for review!

I suggest ignoring the diff for ``chapter.md`` and just reading it directly. :)
Hmm. In database terms, DBDB supports a "transaction", because the process that makes updates can see those updates, but no other process can. Once the updating process commits, all processes can see all the updates. I rewrote this section a bit; I hope it's more clear now (but it can probably still do with improvement).