simplify database fixture to the features we use

The Database fixture has existed for a long time, and has a large
amount of vestigial code that is no longer usable as it's masked by
other test setup. For instance, the conf fixture always sets the db
connection string to in memory db.

Simplify what the Database fixture is doing to just the inmemory
schema caching and reconstruction. Also provide a test case
demonstrating that this both sets up the db correctly, as well as
resets it after we've made changes.

Change-Id: Idc9b587c705c1c1ab4e1173ad1e8e244037284d7
Merge "simplify database fixture to the features we use"