Verify DBWrapper iterators are taking snapshots

The LevelDB docs seem to indicate that an iterator will not take
snapshots (even providing instructions on how to do so yourself).
In several of the places we use them, we assume snapshots to have
been taken.

In order to make sure LevelDB doesn't change out from under us
(and to prevent the next person who reads the docs from having the
same fright I did), verify that snapshots are taken in our tests.

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