L2 sync: Parallel fetching of blocks, classes, state updates

[cchudant]

What is the current behavior?

• Blocks are fetched from the sequencer sequentially.

What is the new behavior?

• L2 sync fetching of blocks, classes, state updates is now parallel.

Does this introduce a breaking change?

N/A (project is in alpha)

Other information

• Usage of futures stream buffered() for parallel fetching,
• Error handling is still a mess, but I've started moving to thiserror-based error handling. I am not opposed to use anyhow instead, but I feel like thiserror is more appropriate here.
• verify_l2 is sync and takes a long time, so I have put it in a tokio blocking task (see https://docs.rs/tokio/latest/tokio/index.html#cpu-bound-tasks-and-blocking-code)
• Rate limits could be handled better

[Trantorian1]

Don't forget to update CHANGELOG.md with a short description of the changes made, as well as linting the project (you can use ./scripts/deoxys lint for that purpose)