simln-lib: handle graceful shutdown during htlc interception

[elnosh]

as a follow-up from #261, if a CriticalError occurs during htlc interception it immediately returns and triggers a simulation shutdown.

Instead of immediately returning, we can trigger a shutdown signal to other interceptors, if any, and wait for them to finish.

[mubarak23]

@elnosh can you give more context to this issue, I like to take it and work on it

[elnosh]

yeah, it's in handle_intercepted_htlc where we intercept HTLCs and pass them on to any Interceptors. If any of those return a CriticalError we would just return immediately. For example, if there are 3 interceptors and one returns a CriticalError but the other two are still doing interceptor stuff, it will return without waiting for the other two to shutdown. Instead we should trigger a shutdown signal to those and wait for them to shutdown before returning.

[mubarak23]

i will work on it and submit a PR