Wait for download thread before destroying da_info

Wait for download thread before destroying da_info

A cancel request doesn't take effect immediately. (__http_gotheaders_cb)
So, below scenario can occur.

1. client-request thread requests cancel a download for a number of reasons. (ex. zombie client)
2. client-request thread is terminated.
3. queue-manager deallocates da_info_list for terminating.
4. download is canceled and then a http thread accesses to da_info for notifying download terminated.
5. heap-use-after-free occurs.

Change-Id: I47a7ef4c4d7ddda5f4594ee30885e3af230c6116