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

Cargo.lock

@@ -598,9 +598,9 @@ 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",

Cargo.toml

@@ -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"