Fix sensor update rate throttling when new sensors are spawned

[iche033]

There is an issue with sensor manager's update loop throttling. When a world is loaded, the sensor manager figures out the max update rate from all the sensors in the world. However, when a new sensor is spawned after the world is loaded, this max update rate is never updated. This causes a problem for example when later spawning a sensor with an update rate higher than all the existing sensors in the world, this new sensor would never hit its target update rate.

This PR adds a flag (global variable, g_sensorsDirty, to avoid breaking ABI) to detect when sensors are added or removed, and recalcuates the sensor manager's max update rate when the flag is dirty.

The bug only affects non-rendering sensors, e.g. imu, ray sensor, etc

To manually test this, you can:

1. download the hokuyo model from gazebo model database, e.g. by inserting it from gazebo's Insert tab on the left
2. create a copy with higher update rate, e.g. copy the model ~/.gazebo/models/hokuyo to ~/.gazebo/models/hokuyo_new and change the update rate in model.sdf and also remember to give it a new name in model.config
3. launch gazebo and insert the original hokuyo model -> check update rate, e.g. Ctrl + T and select the corresponding laser scan topic under LaserStamped msg type
4. insert the new hokuyo model with higher update rate -> check update rate using the same way as before. Verify that the new model is able to reach the target update rate with the changes in this PR.

Signed-off-by: Ian Chen ichen@osrfoundation.org

[ahcorde]

LGTM, just remove the changes in gazebo/rendering/Visual.cc. i will open a PR to fix it separately

[ahcorde]

Fixed brackets #2784