DatabaseState was originally built to help solve some tracking

problems for table indexes. We now need it to track model installation,

and going forward, we'll be using it to track table engines and other

details.
This change updates this class to fix up some small table lookup issues

(ensuring that we always normalize table names), to rename

rescan_indexes() to rescan_tables() (which is a more appropriate

name), and to add a utility function for determining if the database has

a table that backs a given model.
Future changes will update this to track additional information.

Unit tests pass.