Implemented "distinct" in case of "search" and included 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/search '{"query":{"distinct":"foo","from":"test.db-api:1"}}'
[GF-3566] Implemented "distinct" in case of "search"
Open-webOS-DCO-1.0-Signed-off-by: kyouhyun.hwang <kyouhyun.hwang@lge.com>

Change-Id: Ic130c640fda102026a8f46a4712b7402d1472dd2