implemented code to return ItemTablePtr instance
of 'functions' returned from ZABBIX server.

triggerid and the flag that indicates if 'trigger'
information is obtained are stored in the private context.

This implies this class cannot be used form multi-thread.
However, currently it is accessed from multi-thread. It
must be fixed.

<< Tasks to do in the near future >>
- implemented funtions to get the follwoing.
  + items
  + hosts
- parse arguments of --data-store-zabbix and create multiple
  DataStoreZabbix instances.