Figure out how to implement the Cluster.database back reference

[d-bohls]

See existing discussion on this issue in #237.

The Cluster.database back reference is not like other database-related back references because:

1. the Database constructor takes a database path instead of a handle (like other database-related objects)
2. The Database class actually uses the handle as state to determine if the database needs closing.

We should research and decide how to handle this.