[SPARK-6910] [SQL] Support for pushing predicates down to metastore for partition pruning

This PR supersedes my old one #6921. Since my patch has changed quite a bit, I am opening a new PR to make it easier to review.

The changes include-
* Implement `toMetastoreFilter()` function in `HiveShim` that takes `Seq[Expression]` and converts them into a filter string for Hive metastore.
 * This functions matches all the `AttributeReference` + `BinaryComparisonOp` + `Integral/StringType` patterns in `Seq[Expression]` and fold them into a string.
* Change `hiveQlPartitions` field in `MetastoreRelation` to `getHiveQlPartitions()` function that takes a filter string parameter.
* Call `getHiveQlPartitions()` in `HiveTableScan` with a filter string.

But there are some cases in which predicate pushdown is disabled-

Case | Predicate pushdown
------- | -----------------------------
Hive integral and string types | Yes
Hive varchar type | No
Hive 0.13 and newer | Yes
Hive 0.12 and older | No
convertMetastoreParquet=false | Yes
convertMetastoreParquet=true | No

In case of `convertMetastoreParquet=true`, predicates are not pushed down because this conversion happens in an `Analyzer` rule (`HiveMetastoreCatalog.ParquetConversions`). At this point, `HiveTableScan` hasn't run, so predicates are not available. But reading the source code, I think it is intentional to convert the entire Hive table w/ all the partitions into `ParquetRelation` because then `ParquetRelation` can be cached and reused for any query against that table. Please correct me if I am wrong.

cc marmbrus

Author: Cheolsoo Park <cheolsoop@netflix.com>

Closes #7216 from piaozhexiu/SPARK-6910-2 and squashes the following commits:

aa1490f [Cheolsoo Park] Fix ordering of imports
c212c4d [Cheolsoo Park] Incorporate review comments
5e93f9d [Cheolsoo Park] Predicate pushdown into Hive metastore

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