fixes #1374

RETRY events are now deferred until pending transfer is finished
and next RETRY event occurs.

fixes #1400

fixes #1376

fixes #1401

refs #1362 @30m

refs #1362 @20m

- Modified functions for signing XFR queries and SOA query.
  These functions now need the knot_ns_xfr_t structure to work.
  'wire' field should be set to the buffer for the wireformat of
  the query. TSIG data in the structure must be set in order to
  sign the request.
- Modified calls to these functions.

refs #1362 @1h

- Merged a lot of conflicts
	- debug output
	- handling of errors in xfr-handler.c
	- sending error responses from xfr

Conflicts:
	libknot/nameserver/name-server.c
	src/knot/server/xfr-handler.c
	src/knot/server/zones.c

Conflicts:
	libknot/util/error.c
	libknot/util/error.h
	src/knot/server/xfr-handler.c
	src/zcompile/zcompile.c

- Was not adding request MAC size to wire

refs #1360 @1h

refs #1360 @30m

refs #1360 @2h