[SPARK-11117] [SPARK-11345] [SQL] Makes all HadoopFsRelation data sources produce UnsafeRow

This PR fixes two issues:

1.  `PhysicalRDD.outputsUnsafeRows` is always `false`

    Thus a `ConvertToUnsafe` operator is often required even if the underlying data source relation does output `UnsafeRow`.

1.  Internal/external row conversion for `HadoopFsRelation` is kinda messy

    Currently we're using `HadoopFsRelation.needConversion` and [dirty type erasure hacks][1] to indicate whether the relation outputs external row or internal row and apply external-to-internal conversion when necessary.  Basically, all builtin `HadoopFsRelation` data sources, i.e. Parquet, JSON, ORC, and Text output `InternalRow`, while typical external `HadoopFsRelation` data sources, e.g. spark-avro and spark-csv, output `Row`.

This PR adds a `private[sql]` interface method `HadoopFsRelation.buildInternalScan`, which by default invokes `HadoopFsRelation.buildScan` and converts `Row`s to `UnsafeRow`s (which are also `InternalRow`s).  All builtin `HadoopFsRelation` data sources override this method and directly output `UnsafeRow`s.  In this way, now `HadoopFsRelation` always produces `UnsafeRow`s. Thus `PhysicalRDD.outputsUnsafeRows` can be properly set by checking whether the underlying data source is a `HadoopFsRelation`.

A remaining question is that, can we assume that all non-builtin `HadoopFsRelation` data sources output external rows?  At least all well known ones do so.  However it's possible that some users implemented their own `HadoopFsRelation` data sources that leverages `InternalRow` and thus all those unstable internal data representations.  If this assumption is safe, we can deprecate `HadoopFsRelation.needConversion` and cleanup some more conversion code (like [here][2] and [here][3]).

This PR supersedes #9125.

Follow-ups:

1.  Makes JSON and ORC data sources output `UnsafeRow` directly

1.  Makes `HiveTableScan` output `UnsafeRow` directly

    This is related to 1 since ORC data source shares the same `Writable` unwrapping code with `HiveTableScan`.

[1]: https://github.com/apache/spark/blob/v1.5.1/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/parquet/ParquetRelation.scala#L353
[2]: https://github.com/apache/spark/blob/v1.5.1/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/DataSourceStrategy.scala#L331-L335
[3]: https://github.com/apache/spark/blob/v1.5.1/sql/core/src/main/scala/org/apache/spark/sql/sources/interfaces.scala#L630-L669

Author: Cheng Lian <lian@databricks.com>

Closes #9305 from liancheng/spark-11345.unsafe-hadoop-fs-relation.