compress node.data using gzip (to save traffic)

netmon supports gzip compressed node.data as of
https://git.nordwest.freifunk.net/ffnw/netmon/commit/d96458744bfdea81096998f196b833248f98b848



Signed-off-by: Tim Niemeyer <tim.niemeyer@mastersword.de>
- Backported from ffff-next
CC: Dominik Heidler <dominik@heidler.eu>
Signed-off-by: Dominik Heidler <dominik@heidler.eu>
Reviewed-by: Tobias Klaus <tk+ff@meskal.net>