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Also make sure to clean and normalize keys before using them as prefixes.

BREAKING CHANGES:

* `myds.Query(Query{Prefix:"/foo"})` will no longer match "/foobar" (or even
  "/foo"). This is usually what the user expects, we had a tendency to normalize
  "/foo/" to "/foo" (when we clean keys), and many datastores can't efficiently
  search for prefixes that aren't on path-boundaries anyways.
* Given a mount datastore with mounts `["/foo", "/"]`, `myds.Put("/foo", "bar")`
  will put the value to the datastore mounted at "/", not "/foo", as the key "/"
  and "" usually doesn't make sense.

While technically breaking, these changes are much more likely to fix bugs than
they are to break things.