requestor: moved I/O + net to libknot, moved requestor

src/Makefile.am

src/knot/ctl/remote.c

src/knot/nameserver/update.c

src/knot/server/server.h

src/knot/server/tcp-handler.c

src/knot/server/tcp-handler.h

src/knot/server/udp-handler.c

src/knot/server/udp-handler.h

src/knot/zone/events.c

src/knot/zone/zone.c

src/libknot/packet/net.c

src/libknot/packet/net.h

src/libknot/processing/requestor.c

src/libknot/processing/requestor.h

tests/requestor.c