Update tokio requirement from =0.2.0-alpha.6 to =0.2.2

Updates the requirements on [tokio](https://github.com/tokio-rs/tokio) to permit the latest version.
- [Release notes](https://github.com/tokio-rs/tokio/releases)
- [Commits](https://github.com/tokio-rs/tokio/compare/tokio-0.2.0-alpha.6...tokio-0.2.2)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Cargo.toml

@@ -35,7 +35,7 @@ serde = "1.0.102"
 serde_derive = "1.0.102"
 serde-big-array = "0.2.0"
 siphasher = "0.3.1"
-tokio = "=0.2.0-alpha.6"
+tokio = "=0.2.2"
 toml = "0.5.5"
 
 [dependencies.hyper]