Various changes to allow custom commands to sit in front of non-Rattail database.

In particular some of the Continuum versioning code required protection
from import failures, and various assumptions about the "default" database
session were refactored.

Split `db.model` into subpackage.

At least that's all I set out to do, in an effort to improve/fix the import
issues surrounding models in the new batch system. The question of "model
vs API" came up though, so I tried to lean toward model first, API second
when imports happen.

Move some function logic to `db.util`.

Also splits the logic to allow more control at runtime.

Database config/init overhaul.

This contains some not-very-atomic changes:

* Get rid of `get_session_class()` function and return to global `Session`
class approach.
* Primary database `Session` is now configured as part of command
initialization, by default.
* Make `config` object available to subcommands, and `Daemon` instances
(the beginning of the end for `edbob.config`!).
* Add `--stdout` and `--stderr` arguments to primary `Command`. These are
in turn made available to subcommands.
* Overhauled some subcommand logic per new patterns.
* Get rid of a few other random references to `edbob`.
* Added and improved several tests.
* Added ability to run tests using arbitrary database engine.