[Server][FaceRest] Add a new function: getServerIdHostgroupIdNameMap

In the process:
- Use this function in handlerGetTrigger() function.
- Change a member variable of HandlerGetHelper class.
[Server][FaceRest] Use getServerIdHostgroupIdNameMap() function in
handlerGetEvents() function.
[Server][FaceRest] Use getServerIdHostgroupIdNameMap() function in
replyGetItem() function.