Don't have ZeroRPC test fixtures require db fixture.

We weren't actually using this functionality, and we want the zerorpc
fixtures to be scoped to the session (otherwise we get ZeroRPC
socket-in-use errors), whereas the database gets scoped to the
_function_. With pytest, you can't use function-scoped fixture within a
session-scoped fixture.
Clarify sqlalchemy session relationship to db txns.
Remove some database session refreshes from tests.

Now that we're using the READ-UNCOMMITTED isolation mode, these
refreshes to make SELECT return the right data are unnecessary.