[#413,!288] - kea-dhcp6 now uses globals from config back end

[#413,!288] - kea-dhcp6 now uses globals from config back end

src/bin/dhcp6/tests/config_backend_unittest.cc
    TEST_F(Dhcp6CBTest, mergeGlobals) - enabled test

src/lib/dhcpsrv/srv_config.*
    SrvConfig::merge(ConfigBase& other) - invoke merge6()
    SrvConfig::mergeGlobals4() renamed to mergeGlobals()

    SrvConfig::merge6(SrvConfig& other) - new method

src/lib/dhcpsrv/tests/srv_config_unittest.cc
    TEST_F(SrvConfigTest, mergeGlobals6)  - new test