Incorrect sensor update rate in lockstep mode

[iche033]

When two physics based sensors of same type but with different update rates are present in the world, both sensors end up with the same (higher) frame rate.

To reproduce:

1. create two sensors of different update rate, e.g. copy this hokuyo model from gazebo_models to a new model, named hokuyo_100, and change its update rate to 100. and place them into your gazebo models path, e.g. ~/.gazebo/models

2. Launch gazebo and insert the original hokuyo model into the world. Check its update rate by using the Topic Visualization tool (Ctrl +T) and selecting the hokuyo topic under LaserScanStamped msg. The update rate should be 30Hz.

3. Insert the hokuyo_100 model into the world, and see that the update rate for the original hokuyo sensor jumps to 100Hz

[peci1]

Fixed by #2863 in Gazebo 9, so can be closed? However, not fixed in 11!