Improve global state management and concurrency (Context, Controller).

[mcav]

This commit improves and simplifies the way we manage global state, in collaboration with @jedireza.

With this commit:

• Controller coordinates the interaction of services, servers, and events. ContextTrait has been merged into Controller; FoxBox is the concrete implementation of Controller.

  (Why the name change? In practice, Context was acting more like a controller than a context; as was Controller. The term Controller is more precise.)

• We no longer coarsely lock the global state. Previously, we passed around Arc<Mutex<Context>> everywhere (aliased as SharedContext). When we needed to access the global context, we would coarsely lock() the mutex. This is inefficient and cumbersome. Instead, we only lock what we need to.

This allows us to clone and share references to a generic Controller (i.e. <T: Controller>) safely and concurrently.

NOTE: Git did not detect the rename of context.rs to controller.rs; the diff is slightly misleading in that regard. I'd recommend using the split view for easier reading.

[mcav]

r? @fabricedesre

[fabricedesre]

I think you forgot to add that one back when you rebased.

[mcav]

Yeah, good catch.

[fabricedesre]

Very nice!
One nit: Context was meant to carry all the "shared" things. Now we end up exposing things like start_tunnel() and stop_tunnel() to places that should not have access to that. We should break that up in a follow-up.

Please fix the cors header and we'll merge.

[jedireza]

🙌