implemented code to create ItemTablePtr instance
of 'items' based on data returned from ZABBIX server.

This patch retrieves data named 'item' from ZABBIX
server and send it to SQLProcessorZabbix.
Retrieving itself seems to success. However, asura stalls
in the calculation of inner join.

Code to get triggers, functions, hosts, and items
in VirtualDataStoreZabbix, DataStoreZabbix, and
ArmZabbixAPI have similar statements. So we will
extract common procedure to helper functions to
make code simple.

<< Tasks to do in the near future >>
- fix the stall on the inner join.
- make tests for ArmZabbixAPI.
- clean up of VirtualDataStoreZabbix, DataStoreZabbix, and
  ArmZabbixAPI.
- parse arguments of --data-store-zabbix and create multiple
  DataStoreZabbix instances.