Make class for managing g_config_client_list

Make class for managing g_config_client_list

- Requirement:
g_config_client_list is shared memory, so thread safe access for
g_config_client_list is needed.

- Contents:
This patch makes new classes for accessing list of vc_config_client_s
instance. Through this patch, all other functions in this module can
access list of client information in a thread safe way using new class
instance introduced by this patch.
This new class will protect each client information element using mutex,
so the class will assure to prevent race condition.

Change-Id: I2426df83bd763ee3a96334cc64b4b93f6e06a760
Signed-off-by: Suyeon Hwang <stom.hwang@samsung.com>