CarlaDataProvider information when vehicles are spawned from different process.

[Daraan]

The agent relies on information from the scenario_runner's CarlaDataProvider, i.e spawned actors are registered and tracked there.
However, when this spawning is done by a different process, i.e. a different CarlaDataProvider, the agent is not aware of these actors.

Solution gather all actors from the simulator from time to time. For performance reason do this with a flag.
Open question, how to prevent a wrong flag: performance degradation / wrong actor information.

[Daraan]

This is an open issue: carla-simulator/scenario_runner#1079

One could retrieve all actors and add it to the CDP. An additional config flag could be added to check for new actors and include them in each local CDP instance for now.

Idea instead of getting all actors every tick, maybe some message/signal passing can be implemented.

# Low Priority to fix here