[4096] Added client class dictionary to SrvConfig

[4096] Added client class dictionary to SrvConfig

src/lib/dhcpsrv/parsers/client_class_def_parser.cc
    ClientClassDefListParser::commit() - pushes the newly built local
    dictionary to CfgMgr staging

src/lib/dhcpsrv/srv_config.h
src/lib/dhcpsrv/srv_config.cc
    Added client class dictionary member to SrvConfig

src/lib/dhcpsrv/tests/client_class_def_parser_unittest.cc
    TEST_F(ClientClassDefListParserTest, simpleValidList) - now
    verifies that ClientClassDefListParser::commit() pushes its
    local dictionary to CfgMgr staging.

src/lib/dhcpsrv/tests/srv_config_unittest.cc
    Added support to test new client class dictionary member