Implemented "distinct" in case of "find" and inclued modified test code

'Distinct method' remove duplicate objects.
For example,
- input data : {"c", "d", "a", "b", "a", "b", "c", "a", "b", "b", "c", "c" ,"b", "a", "c"}
- expect : {"a", "b", "c", "d"}
Schema
- luna://com.palm.db/find '{"query":"distinct":"foo","from":"test.db-api:1"}}'
[GF-4820] Support group by(distinct) capability on find method
Open-webOS-DCO-1.0-Signed-off-by: Jinyoung Choi <jinyoung76.choi@lge.com>

Change-Id: I71f61b44013d9c7e1408a65716dfc7ce18b778fe
Reviewed-on: https://g2g.palm.com/2445
Reviewed-by: Build Verification
Reviewed-by: Jinyoung Choi <jinyoung76.choi@lge.com>
Tested-by: Jinyoung Choi <jinyoung76.choi@lge.com>
Reviewed-by: Nikolay Orliuk <norluk@luxoft.com>
Reviewed-by: Kyouhyun Hwang <kyouhyun.hwang@lge.com>
Reviewed-by: Roger Stringer <roger.stringer@lge.com>