Resolved the Serialization and Deserialization issues for MultiValued Columns with the specialized "set" and "map" indicators.

Fixed various classes where the KeyValue ColumnType is not handled.
Also added IT tests to cover both Serialization and Deserialization aspects of KeyValue ColumntType by using the external_ids column of Bridge table.

Change-Id: I067bba372da8c02b0befbdb5210dda9068ea397f
Signed-off-by: Madhu Venugopal <mavenugo@gmail.com>