KIJIMR-46. Migrate KeyPassThroughReducer to Kiji MapReduce Lib.

Signed-off-by: Milo <milo@wibidata.com>
KIJIMR-68. MapReduceContext becomes GathererContext.

Signed-off-by: Lee Sheng <lsheng@wibidata.com>
SCHEMA-153. Cleanup after introduction of KijiTable.getRegions()

Signed-off-by: Milo <milo@wibidata.com>