kiji-mapreduce: KIJIMR-251. KijiTableInputFormat reports progress

 - Use the row-key first bytes to compute an approximate progress.
 - Added exclusion for findbugs not picking explicit and expected downcast.

issue: https://jira.kiji.org/browse/KIJIMR-251
review: https://review.kiji.org/r/1419
commit: https://github.com/kijiproject/kiji-mapreduce/commit/1fc980e8ce828ba4f3298a027a20395cd146d0d0