SCHEMA-422. ZooKeeperClient should use release/retain instead of close so that it may be shared

issue: https://jira.kiji.org/browse/SCHEMA-422
review: https://review.kiji.org/r/660
SCHEMA-428. Remove schema validation enabled by the SCHEMA_VALIDATION system property.

issue: https://jira.kiji.org/browse/SCHEMA-428
review: https://review.kiji.org/r/673
SCHEMA-431. Revert changes to Layout.avdl for schema validation.

issue: https://jira.kiji.org/browse/SCHEMA-431
review: https://review.kiji.org/r/674
SCHEMA-426. HBaseDataRequestAdapter.toScan() generates incomplete Scan for fully paged requests.

Columns with paging enabled are now requested with
a ColumnPaginationFilter and a KeyOnlyFilter.

issue: https://jira.kiji.org/browse/SCHEMA-426