Upgrade to futures 0.3.15.

Cargo.toml

@@ -9,7 +9,7 @@ license = "MIT"
 [dependencies]
 bytes = "1.0"
 crossbeam-channel = "0.5.0"
-futures = { version = "0.3.8", default-features = false, features = ["thread-pool"] }
+futures = { version = "0.3.15", default-features = false, features = ["thread-pool"] }
 parking_lot = "0.11.1"
 rand = "0.8.0"
 tokio = { version = "1.7", features = ["macros", "rt-multi-thread", "sync", "time", "parking_lot"] }