Consider `tokio::task::JoinSet` for parallel computation

[ralexstokes]

https://github.com/ralexstokes/mev-rs/blob/7f66dfb4b1f0561365d1d0217e9694b1ca754f8d/mev-boost-rs/src/relay_mux.rs#L168

We likely want to use JoinSet here and anywhere we use this type of pattern for parallelism. The current implementation provides concurrency within one thread but I think this means we are leaving some performance on the table...

[Gerson2102]

I am applying to this issue via OnlyDust platform.

My background and how it can be leveraged

I'm a junior software developer from the Dojo Coding community with contributions to several OnlyDust projects. Including Madara, Cairo native, kakarot-rpc, and Giza. All the projects where I contributed are in Rust, so I have been dealing with this language for a while. You can check my OD profile here: https://app.onlydust.com/u/Gerson2102

How I plan on tackling this issue

This is my first time seeing this project in OD. And it can be my first contribution to the project. I will start by understanding the code related to the issue. If I'm too stuck in this, i will ask questions in your telegram group if you guys have one.