Prepare for deprecation of Authenticator.db

JupyterHub will (not sure how soon) deprecate the `Authenticator.db` property, because the lifecycle of that property is a problem, and Authenticators and Spawners generally shouldn't ever need access to the JupyterHub db, and certainly shouldn't have access to a shared _session_ on that db.

xref: https://github.com/jupyterhub/jupyterhub/issues/3700

### Proposed change

Make NativeAuthenticator's db session independent of the Hub's internal db session. Creating a new Session against the same _database_ (i.e. starting from db_url instead of db) should be fine, especially since NativeAuthenticator's tables are already independent (related to #28, which I think crosses an internal implementation-detail line).

I think this may need some support from JupyterHub to get the necessary info. Worst case: duplicating db_url config from JupyterHub -> NativeAuthenticator. But since the _tables_ are independent, there's actually no reason other than convenience of configuration for the two to share the same db at all, let alone db session.

### Alternative options

- Wait for Authenticator.db to be removed, and stop working.
- Request new `db_session()` factory API to create a _session_ on the db that isn't shared with JupyterHub

### Who would use this feature?

All future deployments of JupyterHub with NativeAuthenticator