Asynchronous start

fixes #249

Two configurable options:
- wait for zones to be loaded before starting (default)
- start server and load zones in the background (asynchronous-start on)

Thread unsafe log

Log subsystem descriptors like open files and facilities weren't reconfigured atomically.
This wasn't a problem in the past (more less, as the events were frozen), but now it is.

The zone file is decoupled from AXFR processing and is written if:
- 'flush' timer is triggered
- immediately after xfer if the 'flush' is set to 0
- on server teardown

The RFC5936/1035 is vague on this with, though technically RAM is
also a "database" and zonefile sync on server teardown is equivalent
to the two-stage model described in section 6.

Configure libidn

(ported 3b5b0f64)

512B limit for a single log line

Interface list could be changed before the threads finish reading
it. This code simply waits for the readers before publishing, but
a smarter queueing is going to be done in #239.

The jobs are removed from the task queue and executed afterwards,
cannot synchronize on the queue only.

- event_dispatch checks for frozen events
- event_cancel must be called outside of the events lock,
as the event itself locks it as well