xdp-tcp: allow configuring outbuf memory limit separately

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/server/xdp-handler.c

src/libknot/xdp/tcp.c

tests/knot/test_confio.c