Fixes #1104 @1h

Conflicts:
	src/tests/libknot/libknot/nsec3_tests.c

Closes #996 @3h

Refs #996 @4h

refs #979

* Although sendmmsg() is not yet available, sendto() is processed in batches.
* Very improved I/O performance due to batch processing and reduced waiting time for socket locks.

refs #603

Conflicts:
	src/knot/server/udp-handler.c

Closes #1084 @2.5h

refs #1103

Adding empty NSID data is completely legal. It is however a matter
of discussion if the function should add the EDNS OPTION or not.

The arguments supported are OK from the viewpoint of the function.

Conflicts:
	src/tests/libknot/libknot/rdata_tests.c

The parse_next_rr_answer() function does not require the pointer
to be NULL.

Closes #1081

+ rdata free test marked TODO.

fixes #1101 @15m

@4h

- Fixed error functions.