Redesign Client class, drop Session class test as it still broken

* Redesign the Client class, the Client class is abstract and split the
  actual implementation into ClientIPv4 and ClientIPv6, both will be
  created with ClientFactory::getClient()
* The new implementation, the IP is constant, no API to change it later.

* Drop Session class test as it still broken due to the Client
  implemenation changes.

Signed-off-by: Neutron Soutmun <neo.neutron@gmail.com>