Refactor TokioRuntimeContext in order to abort all spawned tasks

[ljoss17]

Proposal

The runtime implementation for relayer-next should be updated in order to abort all spawned tasks.

The implementation of TokioRuntimeContext in the relayer-next spawns a new task does not have any method to abort all spawned tasks, https://github.com/informalsystems/hermes/blob/soares/relayer-next/crates/relayer-runtime/src/tokio/context.rs#L24.

One of the use cases for this feature is when running the integration tests with the relayer-next. The current solution only aborts the JoinHandle<()> of the auto_relay() task spawned, https://github.com/informalsystems/hermes/pull/3234/files#diff-50ab12b7fc99fc8b4406c1de2f36eae74380960d5049fe0ec3df3c92c1495f32R421.
Being able to cleanly abort all tasks from the context instead would be better.

One solution could be to keep track of all JoinHandle<()> spawned, https://github.com/informalsystems/hermes/blob/soares/relayer-next/crates/relayer-runtime/src/tokio/context.rs#L95, and have a method abort them and wait for all the tasks to be finished.
A few issues need to be considered for the refactoring:

• If a Vec<Arc<JoinHandle<()>>> is used, note that Arc<JoinHandle<()>> is not aFuture, so it would need to be changed in order to use futures::future::join_all.
• The traits OfaFullRuntime, Spawner will need to be refactored as they currently return Box<dyn TaskHandle> and JoinHandle<()> does not implement Copy or Clone. Creating the TokioTaskHandle, adding an Arc to the vector and returning the TokioTaskHandle is not possible due to a borrow error:

  • let tokio_handle = TokioTaskHandle(join_handle); // Moved join_handle
    let arc_handle = Arc::new(tokio_handle);
    handles.push(Arc::clone(&arc_handle));
    Box::new(tokio_handle)                          // Error caused by previous move
    

• The method spawn() of OfaFullRuntime does not take a mutable reference to self so it would not be possible to simply add a Vec<Arc<JoinHandle<()>>> field to TokioRuntimeContext.

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