Add Clear to DocumentStore API

This is part of a series of commits that will remove Revert/Truncate. I
don't really like the API guarantees these methods are trying to offer,
and I don't have a strong use case for them. However, in tests, we need
to be able to dump DocumentStores easily, so we need some way of
deleting stuff. That's what Clear is for.
Use Clear in test setup

The next part of refactoring DocumentStore is to stop using Revert and
Truncate for test setup.
Remove Truncate/Revert tests

Next, we can take away all the instances of Truncate and Revert in test
bodies, plus any other test clauses that were verifying the behavior of
those methods.
Remove Truncate/Revert from DocumentStore

Finally, we can remove these methods altogether.