[HapProcessStandard] Remove remaining async. tasks in the destructor.
[HapProcessZabbixAPI] Restructure relationship of the classes.

The new class: HapProcessZabbix has the similar class structure
with HapProcessCeilometer. This means that shared code between
them increases. As a result, this patch improves maintainability.
[HapProcessZabbixAPI] Implement fetchItem().
Merge pull request #680 from project-hatohol/item-hapi-zabbix-2

Item hapi zabbix 2