Make root field configurable

[jonaskello]

Today it is hardcoded as ROOT_QUERY which means that even if you are normalizing the result of a mutation the result will still be keyed by ROOT_QUERY. This causes problems when you use __typename and fragments. The __typename of ROOT_QUERY should be Query but if you merge the result of a mutation the __typename of ROOT_QUERY will change to Mutation and fragment on Query will not find the root anymore.

You should be able to send in the key so mutations could get ROOT_MUTATION. And also it is nice to not have it hardcoded of course.