added the following methods.

- DataStore::DataStore(void)
- DataStore::~DataStore()
- DataStoreZabbix::~DataStore()

This patch makes DataStore have a virtual function table.
When the DataStore doesn't a virtual function table,
Building code including dynamic_cast<DataStoreZabbix *>(dataStore) fails.
So the above functions are empty though, this is necessary to implement
with DataStore and DataStoreZabbix.

<< Tasks to do in the near future >>
(P) create DataStoreZabbix instance in mainRoutine().
(Q) remove code to craete ArmZabbixAPI instance in ArmController.
    The ArmZabbixAPI instance will be created in DataStoreZabbix,
    When the above (P) is implemented.
(R) pass command line argument for ArmZabbiXAPI such as IP address etc.
    to DataSotreZabbix.

<< Next milestone >>
showing a 'Overview' 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) VirtualDataStoreZabbix