[#401,!254] kea-dhcp4 now merges CB options into current config

[#401,!254] kea-dhcp4 now merges CB options into current config

Basic merging works, need to add validation against definitions.

src/bin/dhcp4/tests/config_backend_unittest.cc
    TEST_F(Dhcp4CBTest, mergeOptions) - enabled and updated

src/lib/dhcpsrv/cfg_option.*
    CfgOption::merge(CfgOption& other) - new method, conformant
    to others, that merges/updates a config option from another

src/lib/dhcpsrv/srv_config.cc
    SrvConfig::merge4(SrvConfig& other) - modified to
    merge configured options

src/lib/dhcpsrv/tests/cfg_option_unittest.cc
    TEST_F(CfgOptionTest, merge) - new test