fixed to call ArmZabbixAPI::getTriggers() when the table: 'triggers'
is requested to VirtualDataStoreZabbix.

The previous revision periodically executed getTriggers()
as an experiment.

<< Tasks to do in the near future >>
- implements DataStoreZabbix::getTriggers().
- implements VirtualDataStoreZabbix::getTriggers().
- parse arguments of --data-store-zabbix and create multiple
  DataStoreZabbix instances.

<< Next milestone >>
showing a 'Overview' (that is mainly needs triggers data of the server)
of ZABBIX PHP UI. The assumed design of the data stream is the following.

[Zabbix Server] =>
  (A) ArmZabbAPI -> (B) DataStoreZabbix -> (C) VirtualDataStoreZabbix
    -> (D) SQLProcessorZabbix -> (D) FaceMySQLWorker  => ZABBIX PHP client

<< Next next milestone >>
Merge data from two zabbix servers at (C) VirtualDataStore.