Improved performance in get/set operation

Improved performance in get/set operation

- Add hashtable in buxton daemon to reduce db access.
  Add hashtable in client for repeated get operation on the same key.
  Send notis to client after sending result of set operation.

- Brief profiling data
 +---------------------------------------------------+
 | - Gerenal set/get operation                       |
 | vconf set average : 5.420375 ms -> 3.430805 ms    |
 | vconf get average : 1.721345 ms -> 0.592360 ms    |
 |                                                   |
 | - Repeated get operation on the same key          |
 | vconf get average : 1.721345 ms -> 0.010990 ms    |
 +---------------------------------------------------+

Change-Id: Ib7ffd75dbd3bb11b76e9729f4177605848c47dfe
Signed-off-by: Jiwoong Im <jiwoong.im@samsung.com>