[server][FaceRest] Support to monitor "New Value Per Second"

In this implementation, NVPS is calculated when items are fetched,
then they are stored into 'servers' table.
FaceRest just returns stored the value of nvps.

Currently, there is a known issue that the value of nvps is different between zabbix and hatohol (I've commented details at #104), so I've reported  as [ZBX-7872](https://support.zabbix.com/browse/ZBX-7872) . it may be fixed in the future.