Refactor common sensor setup interactions

[mjcarroll]

This addresses a bug uncovered when using the Reset implementation with the ForceTorqueSensor.

Not all sensor system implementations were updated as part of https://github.com/ignitionrobotics/ign-gazebo/pull/1281, one of which being the ForceTorqueSensor. This makes the reset behavior incorrect and the sensor won't be respawned.

As part of this, I'm also experimenting with refactoring out the common functionality that is boilerplate in any GzSensors system implementation. To view the diff for a single system that was already updated, see https://github.com/ignitionrobotics/ign-gazebo/commit/20b15b619a3d67d607d8bbcb238b34689a8b9b9c

Once complete, this should close out: https://github.com/ignitionrobotics/ign-gazebo/issues/797 as all sensors will be updated.