SCHEMA-139. Fix KijiTableKeyValueDatabase.putValue() return type.

@jhlch 

Return the implementing subclass type (e.g., KijiMetaTable), not
KijiTableKeyValueDatabse itself.

Adds new unit tests. The ``testSameMetaTableOnPut` case would not have succeeded before SCHEMA-139. We caught a bug! :)

tested w/ mvn clean verify.