fixes #99

Like example.com..zone

This makes partial-closed connections discovery faster than waiting
full interval for the watchdog sweep.

refs #91

Reverted to usual recvmmsg/sendmmsg availability check.

refs #82

fixes #82

Treated as hostname otherwise.

- Nighlty tests would sometimes crash, when zones were not bootstrapper properly.

Also some log messages cleanup.

Also CID#1047013

refs #71, #65

The reasons are discussed in #71.
Namely:
- transfers are self-paced, TCP clients are divided among threads
- due to the nature of DNS, most of the connections should be active
- if not, it is probably an attack, long idling connections are disconnected
- even then the performance degradation is not so severe

fixes #71

Otherwise it would finish all pending queries, some of which may have
connection already dropped. This usually isn't a problem, it just
presents a longer wait for the timeouts.

Existing issues must not be treated as expired, since that might
trigger bootstrap requeue or NOTIFY event.

This is because the query should be sent as soon as the socket is
writable as well.

refs #65

It was possible for the previous implementation to sometimes overshoot
the hard limit by a few connections, this yielded very undesirable results
as the master starts dropping connections and forcing slave to reschedule
transfers.

Asynchronous connect is also faster as other pending transfers may be processed
during the connection phase.

refs #65

refs #65

To do:
* Disabled epoll/kqueue backend as there is something broken for write events

refs #65

    - This assert made no sense, flow can get into this function even if previous is set. In fact, previous is used in the function that is called just after said assert.
Previous was set to NULL in previous implementation, which was wrong as well.