title: Uniqueness constraint and indexing issue
body: <p>I must be doing something incorrectly, or have a misconceptions about constraints and indexing.  I have the following:</p>
  
  <pre><code>CREATE CONSTRAINT ON (u:User) ASSERT u.user_id IS UNIQUE
  </code></pre>
  
  <p>and</p>
  
  <pre><code>CREATE INDEX ON :User(user_id)
  </code></pre>
  
  <p>I've tried alternating the order, but regardless, I'm getting:</p>
  
  <pre><code>Neo.ClientError.Schema.ConstraintAlreadyExists
  </code></pre>
  
  <p>or</p>
  
  <pre><code>Neo.ClientError.Schema.IndexAlreadyExists
  </code></pre>
  
  <p>depending on the ordering.  </p>
  
  <p>I don't understand why I wouldn't be able to do this.  I want look-ups to be fast for a <code>user_id</code>, which is why I'm indexing, and I also want to make sure that <code>user_id</code> is unique, which is why I have a constraint.  </p>
  
  <p>What am I misunderstanding?  How should I go about doing this?</p>