SCHEMA-598. Provide a static empty KijiDataRequest

issue: https://jira.kiji.org/browse/SCHEMA-598
review: https://review.kiji.org/r/1320
SCHEMA-588. ColumnReaderSpec: a specification of read properties on a column.

issue: https://jira.kiji.org/browse/SCHEMA-588
review: https://review.kiji.org/r/1252
SCHEMA-591. Plug ColumnReaderSpec in KijiDataRequest.

issue: https://jira.kiji.org/browse/SCHEMA-591
review: https://review.kiji.org/r/1255
SCHEMA-606. Plug ColumnReaderSpec into KijiTableReader

issue: https://jira.kiji.org/browse/SCHEMA-606
review: https://review.kiji.org/r/1451
SCHEMA-557. Move away from HTablePool

issue: https://jira.kiji.org/browse/SCHEMA-557
review: https://review.kiji.org/r/1449