Negative integer values in clock columns

Clock columns are interger signed types, but having such types allows to enter negative numbers. With zabbix_sender I can "accidentally" send negative clock values and write them in history_uint.clock and trends_uint.clock. Although they somehow correct those clock values to -1, but this is still incorrect. We should not allow to import such values and give out an error that data is corrupted. Additionally we could maybe even change column type to unsigned, to prevent such behaviour in the future.