Downgrade back to Tokio 0.2

[romac]

Crate

relayer

Summary

Let's get out of dependency hell and downgrade back to Tokio 0.2 for the low price of a Mutex lock when blocking on async task.

Problem Definition

The upgrade to Tokio 0.3 was poorly planned by myself, and done way to eagerly, which landed us in a dependency hell.

Proposal

Downgrade Tokio to v0.2, and wrap the tokio Runtime in a Arc<Mutex<_>>.

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