SCHEMA-140. Clean up avro definition of MetaTableBackup.

Signed-off-by: Milo <milo@wibidata.com>
Added release/retain semantics to KijiTable

 - removed AbstractKijiTable.
 - modified kiji-schema build to fail if resources are not closed in tests.
 - KijiTable.close() is deprecated.
 - HBaseKijiTable is final.