src/bin/dhcp4/tests/config_backend_unittest.cc
    TEST_F(Dhcp4CBTest, mergeGlobals) - enabled/revamped

src/lib/dhcpsrv/tests/cfg_option_def_unittest.cc
    TEST(CfgOptionDefTest, merge) - new test

src/lib/dhcpsrv/cfg_option_def.*
    CfgOptionDef::merge(CfgOptionDef& other)  - new method
    which merges option definitions

Removed const from "other" parameter in in the following merge methods:

    CfgSharedNetworks4::merge(CfgSharedNetworks4& other)
    CfgSubnets4::merge(CfgSharedNetworks4Ptr networks, CfgSubnets4& other);
    SrvConfig::merge(ConfigBase& other)
    SrvConfig::merge4(SrvConfig& other)
    SrvConfig::mergeGlobals4(SrvConfig& other)
    ConfigBase::merge(ConfigBase& other)