Update Tokio to 0.2.21 (#46)

As Tokio version 0.2.21 contains a fix in the broadcast channel that
removes a memory leak in mini-redis, I don't think 0.2.20 should be
considered the minimum supported version, even if the codebase compiles
with that version.

Refs: #38

Author: Darksonn

Cargo.lock

@@ -25,7 +25,7 @@ async-stream = "0.2.1"
 atoi = "0.3.2"
 bytes = "0.5.4"
 structopt = "0.3.14"
-tokio = { version = "0.2.20", features = ["full"] }
+tokio = { version = "0.2.21", features = ["full"] }
 tracing = "0.1.13"
 tracing-futures = { version = "0.2.3" }
 tracing-subscriber = "0.2.2"