SCHEMA-310. Ensure pooled tables are released when marked idle.

When the idle connection cleanup thread releases pooled table objects, it was
originally releasing the underlying HBase table instead of the decorated pooled
table which breaks encapsulation.

Reviewed by: taton
SCHEMA-311. Renamed Connection to PooledKijiTable.

Renamed the internal Connection class to PooledKijiTable. This better represents
the decorating of a KijiTable for pooling purposes.

Reviewed by: taton
SCHEMA-303. KijiRowFilter should have a proper serialization/deserialization

 - Row filters are now serialized to/from JSON.

issue: https://jira.kiji.org/browse/SCHEMA-303
review: https://review.kiji.org/r/128/
Update release notes for SCHEMA-303.