refs #755

XFR handler uses event queue for receiving requests.
Each XFR handler thread asynchronously listens for events like UDP handler.

Commit refs #752.

Commit refs #774.

refs #755

- Pointer must be less than 16383.

refs #755

- Size was 'short' which is not enough for 64k packet. Changed
  to size_t to be compatible with the rest of the server.

TODO: there is still some issue with TCP giving more than one
      query to parse.

refs #774

- The file was not compiled at all.
- Had to rename it to dnslib-error.c, as it is impossible to have
  more source files with the same name in the project (Makefile
  issues).

To do:
* XFR handling is synchronous, implement async XFR handler.
* Cleanup in UDP

Commit res #756.

refs #755

- Was replacing the response structure with a new one.

refs #749

Commit refs #756.

- Reused Marek's old code.

refs #755

refs #755

refs #755

- If the response is full, send it, clean the structure and
  continue with the last RRSet that was to be added.

refs #755

- Takes query ID instead of the whole query wire format.

refs #755

refs #755

- Minor change to ns_add_rrsigs() - checks if the wildcard checking
  has sense (name given).
- ns_axfr_from_node() should add all RRSets from node and their
  RRSIGs. Does not add SOA RRSet.

refs #755

refs #754

- All XFR parameters are now in one structure which is passed
  to the functions.

TODO: move the type definitions somewhere else

refs #754, #755

- AXFR will use the tree traversing functions to get data from all
  nodes.
- Error in some node is carried on in the data passed to the
  function. Not sure if this works.

refs #755

- Added function ns_axfr() which finds zone to answer from.
- Added stub of ns_axfr_from_zone() which should do the actual
  AXFR answering.
- Using RCU when dereferencing pointer to zone db.

refs #755

refs #754

- Added a separate function to nameserver to parse query and get
  its type.
- Added special type for query types to dnslib/consts.h.

WARNING: The rest of the API is not modified yet to support the
         pre-parsed query, so you must still call
         ns_answer_request() with the unparsed query.

refs #750, 749

TODO: later remove the initialization of space for wireformat from
      dnslib_response_init(), so that it is not reallocated on a
      regular basis.