Upgrade deps to Tokio 1.0

[adizere]

Crate

relayer and cli

Summary

We previously attempted to upgrade to tokio 0.3 which lead to a dep hell so that change was reverted.

It seems the ecosystem (tonic, hyper) are now ready for Tokio 1.0, so we'll consider doing that soon. This would help with (1) a better (cleaner) runtime, and (2) cleaner use of block_on which is linked to performance issues (more context in the comment linked below).

We cannot remove it everywhere as depending on the caller stack we may end up with runtime-in-a-runtime error. Let's hope this will be cleaner when we migrate to tokio1.0

Originally posted by @ancazamfir in #515 (comment)

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