[3797] Added support for Control Channel to DHCPv6

[3797] Added support for Control Channel to DHCPv6

src/bin/dhcp6/ctrl_dhcp6_srv.cc
    ControlledDhcpv6Srv::ControlledDhcpv6Srv()
        added CommandMgr init and handler registration

    ControlledDhcpv6Srv::~ControlledDhcpv6Srv() {
        added CommandMgr shutdown and handler deregistration

src/bin/dhcp6/json_config_parser.cc
    - createGlobal6DhcpConfigParser()
        added support for "control-socket" element

    - configureDhcp6Server()
        added logic to configure CommandMgr based on
        control-socket configuration element

src/bin/dhcp6/tests/ctrl_dhcp6_srv_unittest.cc
    - UnixControlClient  - new class that acts as UnixCommandSocket client    - CtrlChannelDhcpv6SrvTest - new test fixture for testing a DHCPv6 server
    with a Control Channel

    - Added the following tests:
    TEST_F(CtrlDhcpv6SrvTest, commandsRegistration)
    TEST_F(CtrlChannelDhcpv6SrvTest, controlChannelNegative)
    TEST_F(CtrlChannelDhcpv6SrvTest, controlChannelShutdown)
    TEST_F(CtrlChannelDhcpv6SrvTest, controlChannelStats)