Sync time stepping of physics and sensors

[mabelzhang]

A continuation of this PR on BitBucket to sync the time stepping in physics and sensors, so that it is possible for sensors to achieve a strict update rate, regardless of the speed of the hardware:
https://bitbucket.org/osrf/gazebo/pull-requests/2502/make-sure-cameras-fps-is-strictly-applied

While the PR above only syncs cameras, we want to extend it to all sensors. Backward compatibility and API/ABI breakage need to be checked on the original PR.

Some of the API changes it requires had been pulled out and merged in these two PRs for Gazebo 11:
https://bitbucket.org/osrf/gazebo/pull-requests/3180/use-direct-api-call-to-scene-setposemsg-to/diff
https://bitbucket.org/osrf/gazebo/pull-requests/3184/add-virtual-methods-to-sensor-and-other/diff

The BitBucket branch had been pulled to respect_fps branch, which has been updated with the API changes above. Note that the When ready lines in PR 3180 above for setting pose directly need to be uncommented in order for the strict rate feature to work.

Custom XML namespace <xmlns:strict_rate> is to be used for specifying the Boolean flag for a strict update rate.
Usage example:

<sensor name="camera_sensor" type="camera" xmlns:strict_rate="https://github.com/osrf/gazebo/issues/2736">
  <strict_rate:value>true</strict_rate:value>
</sensor>

The flag defaults to false.