Adding MacOS support

[zhengc84]

Adding MacOS compatibility to run geoscenario server. MacOS defaults to "spawn" multiprocessing which pickles objects, but this causes an issue as some objects built by boost cannot be pickled. An alternative solution of using "fork" as well as threading to run the SDV, traffic, as well as Dash is implemented.

The features of basic geoscenarioserver should run the same, producing same test results. The UI should also look identical.

The setup for MacOS will be different from normal geoscenarioserver.

how will MacOS compatibility work with Geoscenario-orchestrator-prod?

[mantkiew]

I have cleaned up and minimized the changes by removing everything that's not needed or accidental.

Here's the design:

1. GSServer runs traffic in a separate thread
2. SDVPlanners run in separate processes just as before but forcing the "fork" method (fork is default on Linux, so no change, but it overrides the default "spawn" on MacOs).
3. GSServer runs dashboard in it's own (main) thread

However, the performance is worse than on master. Good test is test_scenarios/gs_ringroad_stress_loop.osm.
In that scenario, on master, we see consistent dash rate 0.025 as configured and

On this branch we see very inconsistent dash rate jumping to very high values and

As you can see, on this branch the single process (dash and traffic threads) saturates the CPU (130% utilization), whereas, on master the load is spread over two processes: one 50% and one 80%.

[mantkiew]

We cannot merge this as the run-time performance is not good. But it's a good experiment. Let's keep it for now unmerged.