Different log system defaults policy.

1. when no 'log' section is configured, defaults apply
2. when 'log' section is configured & empty, no logging occurs
3. stderr & syslog do not automatically log fatal errors

fixes #61
No need to do a full tree search when node is in the zone.

- Should be substantially faster for "hit" lookups.
Merge branch 'log_defaults_fix' of /git/repositories/knot
Added missing return check.
Merge branch 'lookup-fix' of /git/repositories/knot
Small fix in sem checks - do not continue if no nsec3 rrset exists.
Added missing return to semantic checks.
Remove unused define and reformat
Removed faulty assert.

    - 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.
Added support for different events (READ/WRITE) for fdset.

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

refs #65
Added support for MSG_NOSIGNAL to stop signalling on broken conns.

refs #65
Asynchronous connect for transfers, fixed self pacing.

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
Raise 'connecting' flag for UDP requests as well.

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

refs #65
Fixed cancelling of pending tasks on xfer handler stop.

Existing issues must not be treated as expired, since that might
trigger bootstrap requeue or NOTIFY event.
Thread state should be checked after each request.

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.
Single poll-based lighter weight fdset implementation.

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
Updated to the new fdset API and implemented precise pending xfer counter.

refs #71, #65
Fixed merge request #32 remarks.
Merge branch 'fdset_squash' of /git/repositories/knot
Correct processing of incoming TCP in case of exhausted memory.

Also CID#1047013