Overlapping VehiclePositions updates

[ciolt]

Continuation of #2. This time, there's 2 generators firing at the same time at around the same interval.

Make it so that there's a global register(?) so that the generators fire only one at a time for any given window of time. Might require a check in the exec-vp script.

Should also bump up timeout time to 30 seconds instead of 20.