[SPARK-6595][SQL] MetastoreRelation should be a MultiInstanceRelation

Now that we have `DataFrame`s it is possible to have multiple copies in a single query plan.  As such, it needs to inherit from `MultiInstanceRelation` or self joins will break.  I also add better debugging errors when our self join handling fails in case there are future bugs.

Author: Michael Armbrust <michael@databricks.com>

Closes #5251 from marmbrus/multiMetaStore and squashes the following commits:

4272f6d [Michael Armbrust] [SPARK-6595][SQL] MetastoreRelation should be MuliInstanceRelation