Reimplemented XFR handler to support queueing.

Requests are now scheduled in FCFS manner, while there
can be a fixed amount of pending requests from N threads.
Currently the limit is set to 10 parallel requests to honor
BIND9 default setting, but should be configurable.

To do:
- Fetch pending request state for 'zonestatus' command
- As of now, there can be a multiple NOTIFY tasks,
  would it be a problem?
- Cleanup of the xfr_answer()
- Check for bugs