Support fast merge in the storage plugin interface.

[jealous]

Allow the plugin developer to implement a fast merge which will be invoked when:

• Encryption is disabled.
• Compression is disabled or the compression codec supports concatenation of serialized streams.
• The spark.shuffle.unsafe.fastMergeEnabled option is true.
• The plugin supports fast merge.

Note the performance of this fast merge function could seriously impact the performance of Spark SQL joins with multiple spills.