Convert ServiceImpl namespace to a class

Convert ServiceImpl namespace to a class

This class will be used in future patches:
- to hold ownership of Cynara and PrivilegeDb objects
- to polymorph into basic, slave and off-line versions
- to synchronize multiple concurrent clients (multi-threading is coming)

Change-Id: I54f0ecda081db17350209c3e56debd91927e364e
Signed-off-by: Rafal Krypa <r.krypa@samsung.com>