Zabbix

A.F.....S. [ZBXNEXT-1689] implemented server-side value cache

Before the change:
- Information about last and previous item values was stored in the items tables which required frequent updates and caused table availability problems.

After the change:
- Zabbix server stores last item values in the value cache. Frontend retrieves those values directly from the history tables. If the history storage period is set to 0, item last value information will not be available in the overview, latest data pages and the {ITEM.LASTVALUE} macro will not work. Item last check dates will no longer be displayed for unsupported items. Item change value will no longer be displayed in the latest data page when an item receives the first value. 
- Item lastclock, lastns, lastvalue, prevvalue and prevorgvalue columns have been dropped. 
- Item API prevorgvalue property was removed, lastclock, lastns, lastvalue and prevvalue properties are still supported. Host.get with_historical_items and hostgroup.get with_historical_items parameters were removed.
- The item queue is now requested from the Zabbix server instead of being calculated on the frontend side.