[SPARK-5182] [SPARK-5528] [SPARK-5509] [SPARK-3575] [SQL] Parquet data source improvements

This PR adds three major improvements to Parquet data source:

1.  Partition discovery

    While reading Parquet files resides in Hive style partition directories, `ParquetRelation2` automatically discovers partitioning information and infers partition column types.

    This is also a partial work for [SPARK-5182] [1], which aims to provide first class partitioning support for the data source API.  Related code in this PR can be easily extracted to the data source API level in future versions.

1.  Schema merging

    When enabled, Parquet data source collects schema information from all Parquet part-files and tries to merge them.  Exceptions are thrown when incompatible schemas are detected.  This feature is controlled by data source option `parquet.mergeSchema`, and is enabled by default.

1.  Metastore Parquet table conversion moved to analysis phase

    This greatly simplifies the conversion logic.  `ParquetConversion` strategy can be removed once the old Parquet implementation is removed in the future.

This version of Parquet data source aims to entirely replace the old Parquet implementation.  However, the old version hasn't been removed yet.  Users can fall back to the old version by turning off SQL configuration `spark.sql.parquet.useDataSourceApi`.

Other JIRA tickets fixed as side effects in this PR:

- [SPARK-5509] [3]: `EqualTo` now uses a proper `Ordering` to compare binary types.

- [SPARK-3575] [4]: Metastore schema is now preserved and passed to `ParquetRelation2` via data source option `parquet.metastoreSchema`.

TODO:

- [ ] More test cases for partition discovery
- [x] Fix write path after data source write support (#4294) is merged

      It turned out to be non-trivial to fall back to old Parquet implementation on the write path when Parquet data source is enabled.  Since we're planning to include data source write support in 1.3.0, I simply ignored two test cases involving Parquet insertion for now.

- [ ] Fix outdated comments and documentations

PS: This PR looks big, but more than a half of the changed lines in this PR are trivial changes to test cases. To test Parquet with and without the new data source, almost all Parquet test cases are moved into wrapper driver functions. This introduces hundreds of lines of changes.

[1]: https://issues.apache.org/jira/browse/SPARK-5182
[2]: https://issues.apache.org/jira/browse/SPARK-5528
[3]: https://issues.apache.org/jira/browse/SPARK-5509
[4]: https://issues.apache.org/jira/browse/SPARK-3575

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Author: Cheng Lian <lian@databricks.com>

Closes #4308 from liancheng/parquet-partition-discovery and squashes the following commits:

b6946e6 [Cheng Lian] Fixes MiMA issues, addresses comments
8232e17 [Cheng Lian] Write support for Parquet data source
a49bd28 [Cheng Lian] Fixes spelling typo in trait name "CreateableRelationProvider"
808380f [Cheng Lian] Fixes issues introduced while rebasing
50dd8d1 [Cheng Lian] Addresses @rxin's comment, fixes UDT schema merging
adf2aae [Cheng Lian] Fixes compilation error introduced while rebasing
4e0175f [Cheng Lian] Fixes Python Parquet API, we need Py4J array to call varargs method
0d8ec1d [Cheng Lian] Adds more test cases
b35c8c6 [Cheng Lian] Fixes some typos and outdated comments
dd704fd [Cheng Lian] Fixes Python Parquet API
596c312 [Cheng Lian] Uses switch to control whether use Parquet data source or not
7d0f7a2 [Cheng Lian] Fixes Metastore Parquet table conversion
a1896c7 [Cheng Lian] Fixes all existing Parquet test suites except for ParquetMetastoreSuite
5654c9d [Cheng Lian] Draft version of Parquet partition discovery and schema merging