Remove Spawner.db dependency to address access deprecation

[tgmachina]

This is meant to fix #306

I first noticed the issue in my PR #313 both developing locally (I downgraded JupyterHub to get tests to pass). I decided to try and fix this to help my other PR along.

My changes at a high-level:

• Modify new_spawner to instantiate a sufficiently patched spawner directly, bypassing the orm user instantiation
• Convert new_spawner to a contextmanager to drop patches after test completion
• Created a mock user that has the minimum api to allow the spawner to work
• Dropped the use of event_loop (I'm guessing this was perhaps necessary when using the orm instantiated spawner?)
• Finally, removed self.db.commit() from BatchSpawnerBase.start: I didn't see why it would be necessary to have and having written other spawners myself, I don't recall ever needing it (open to clarification on this point)

To the last point about just removing self.db.commit() rather than checking for a minimum version, I also remembered reading this in the upstream jupyterhub issue:

...one impediment to that is assumptions of a persistent db session on various objects. Authenticators and Spawners have this, but shouldn't.

I assumed @minrk's comment was true for all versions.