conf: add client TFO option 'server.tcp-fastopen'

doc/man/knot.conf.5in

doc/reference.rst

src/knot/conf/base.c

src/knot/conf/base.h

src/knot/conf/schema.c

src/knot/conf/schema.h

src/knot/events/handlers/notify.c

src/knot/events/handlers/refresh.c

src/knot/events/handlers/update.c

tests/knot/test_confio.c