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.