Add `db_uri` attribute to Engine.

And have the `destination` reference it.
Make db_uri read/write.

But don't merge it with the default when setting the value.
Still merge when the value is set via the constructor, though. At
least for now.
Switch to db_uri in the SQLite engine.
Replace db_name with db_uri in SQLite engine.

One test failing, but I think I am going to reverse some of this stuff, and
move the URI into the base engine class.