Feature: Controlled simulation shutdown

[okjodom]

When shutdown is triggered, we should have a more controlled shutdown of the simulation.

Proposal from @sr-gi

• First, you stop the producers, so no new payments are passed down the pipeline
• Then you wait for whatever we feel is reasonable for inflight payments to complete

[carlaKC]

Then you wait for whatever we feel is reasonable for inflight payments to complete

Not sure that we'll be able to pick a value for this - for example, testing a scenario where payments get stuck for 2 weeks: where do we shutdown?

I think that a more user-oriented way to approach this could be to have a --wait-for-completion option that they can opt-in to explicitly wait for all actions to exit once they've triggered shutdown (rather than our picking a time that may be opaque to an end user).