Upgrade to latest tokio

[dconnolly]

Scope

This ticket is only for upgrading tokio, and any required Zebra changes.

The upgrade PR must not include any optional refactors or new tokio features.
This helps reduce the risk of bugs during the upgrade.

Motivation

Zebra is a major version behind the latest tokio.

Stable tokio may help us keep the network stack and async constructions more maintainable.

Suggested Solution

1. Upgrade all zebra crates to use tokio 1.+, and update all APIs, types, etc accordingly.
2. We might need to remove some git dependencies to do these updates, see Remove git dependencies on Hyper, Metrics, Tower #1086.
3. Remove the exceptions for these crates in deny.toml.

Alternatives

Don't upgrade, stay on 0.3.* tokio

PRs and Reviewers

• Manually pin Sleep futures #2914
• Update Tower dependency to 0.4.9 #2918 - @dconnolly & @teor2345
• Update Tokio to version 1.12.0 #2920 - @dconnolly
• Use tokio-stream crate to replace usages of Tokio types that were Streams #2922 - @oxarbitrage
• Use broadcast::Receiver::recv instead of next #2933
• Handle Semaphore::acquire() error #2934 - @teor2345
• Update some zebra-network dependencies #2935 - @conradoplg
• Upgrade Hyper to version 0.14 #2936 - @oxarbitrage
• Update metrics to version 0.17 #2937 - @teor2345
• Update Sentry to version 0.23.0 #2954 - @dconnolly