[#3770] Finished MySql v4, Expanded v4 UTs

[#3770] Finished MySql v4, Expanded v4 UTs

/src/hooks/dhcp/mysql/mysql_cb_dhcp4.*
/src/hooks/dhcp/mysql/mysql_cb_impl.cc
    Added client classes to where clauses as needed

/src/hooks/dhcp/mysql/tests/mysql_cb_dhcp4_unittest.cc
    TEST_F(MySqlConfigBackendDHCPv4Test, globalOption4WithClientClassesTest)
    TEST_F(MySqlConfigBackendDHCPv4Test, sharedNetworkOption4WithClientClassesTest)
    TEST_F(MySqlConfigBackendDHCPv4Test, subnetOption4WithClientClassesTest)
    TEST_F(MySqlConfigBackendDHCPv4Test, poolOption4WithClientClassesTest) - new tests

/src/hooks/dhcp/pgsql/tests/pgsql_cb_dhcp4_unittest.cc
    TEST_F(PgSqlConfigBackendDHCPv4Test, globalOption4WithClientClassesTest)
    TEST_F(PgSqlConfigBackendDHCPv4Test, sharedNetworkOption4WithClientClassesTest)
    TEST_F(PgSqlConfigBackendDHCPv4Test, subnetOption4WithClientClassesTest)
    TEST_F(PgSqlConfigBackendDHCPv4Test, poolOption4WithClientClassesTest) - new tests

/src/lib/dhcpsrv/testutils/generic_cb_dhcp4_unittest.cc
    New tests and functions