Fix block aquire data by req fetch item 4

These patches fix the problem in which ArmPlugin may stall.
Previously On-demand Item Fetch is done synchronously on the AMPQ's receiver thread. This waits for the completion of the fetching task on the GLib event loop. If acquireData() communicates with the Hatohol server such as calling getTimeOfLastEvent() at this time, the reply is blocked.

These patch changes the execution of fetching items to async. way.