Preparing for Melodic sync 2019-09-06

I’m not sure when you are planning for the next Melodic sync. There might be a potential regression in rviz (https://github.com/ros-visualization/rviz/commit/d171d823965e456c6fec2286d6fa10541c84f1bb#commitcomment-35201848). Please hold back the next sync until this is resolved.