[server][CacheServiceDBClinet] Disable `new CacheServiceDBClinet`

fix #239

The current implementation causes build warning on CentOS 6:

      CXX    CacheServiceDBClient.lo
    In file included from CacheServiceDBClient.cc:25:
    CacheServiceDBClient.h: In static member function 'static void* CacheServiceDBClient::operator new(size_t)':
    CacheServiceDBClient.h:52: warning: 'operator new' must not return NULL unless it is declared 'throw()' (or -fcheck-new is in effect)

We can just remove implementation of `operator new()` to disable `new
CacheServiceDBClient`. If someone use `new CacheServiceDBClient`,
compiler reports the following error:

    error: ‘static void* CacheServiceDBClient::operator new(size_t)’ is private

So we can detect `new CacheServiceDBClient` on build time.