conf: add zone.zonefile_sync value -1 to never flush

doc/man/knot.conf.5in

doc/reference.rst

src/knot/conf/scheme.c

src/knot/ctl/remote.c

src/knot/zone/events/handlers.c

src/knot/zone/events/replan.c

src/knot/zone/zone.c

src/knot/zone/zone.h