Upgrade Tokio to version 0.3.0

[romac]

Already implemented in #401.

Crate

relayer, relayer-cli, modules

Summary

Upgrade Tokio to version 0.3 to benefit from it's improved Runtime to facilitate integration of the upcoming chain runtime.

Problem Definition

The upcoming chain runtime needs to bridge the gap between async and sync code.

The improved Runtime in Tokio 0.3 would help us implement a better chain runtime, thanks to its ability to be wrapped in an Arc, and shared between threads and components while still allowing to block_on futures.

Proposal

Upgrade Tokio to version 0.3.

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