[SPARK-6450] [SQL] Fixes metastore Parquet table conversion

The `ParquetConversions` analysis rule generates a hash map, which maps from the original `MetastoreRelation` instances to the newly created `ParquetRelation2` instances. However, `MetastoreRelation.equals` doesn't compare output attributes. Thus, if a single metastore Parquet table appears multiple times in a query, only a single entry ends up in the hash map, and the conversion is not correctly performed.

Proper fix for this issue should be overriding `equals` and `hashCode` for MetastoreRelation. Unfortunately, this breaks more tests than expected. It's possible that these tests are ill-formed from the very beginning. As 1.3.1 release is approaching, we'd like to make the change more surgical to avoid potential regressions. The proposed fix here is to make both the metastore relations and their output attributes as keys in the hash map used in ParquetConversions.

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Author: Cheng Lian <lian@databricks.com>

Closes #5183 from liancheng/spark-6450 and squashes the following commits:

3536780 [Cheng Lian] Fixes metastore Parquet table conversion