Bump tokio from 0.2.20 to 0.2.21

Bumps [tokio](https://github.com/tokio-rs/tokio) from 0.2.20 to 0.2.21.
- [Release notes](https://github.com/tokio-rs/tokio/releases)
- [Commits](https://github.com/tokio-rs/tokio/compare/tokio-0.2.20...tokio-0.2.21)

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

Cargo.lock

@@ -1702,9 +1702,9 @@ dependencies = [
 
 [[package]]
 name = "tokio"
-version = "0.2.20"
+version = "0.2.21"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "05c1d570eb1a36f0345a5ce9c6c6e665b70b73d11236912c0b477616aeec47b1"
+checksum = "d099fa27b9702bed751524694adbe393e18b36b204da91eb1cbbbbb4a5ee2d58"
 dependencies = [
  "bytes",
  "fnv",