SCHEMA-322. Restrict KijiRowData.getPager(family) to list the qualifiers in the family

- adds StripValueColumnFilter (cloned from StripValueRowFilter).
- update KijiPager documentation for map-type families.
- implement pager for map-type families, to retrieve qualifiers only.
- remove obsolete tests.

issue: https://jira.kiji.org/browse/SCHEMA-322
review: https://review.kiji.org/r/283