consensus: Make future handling a little nicer

Part of #428.

Cargo.lock

@@ -2127,6 +2127,7 @@ version = "0.1.0"
 dependencies = [
  "color-eyre",
  "eyre",
+ "futures-util",
  "tokio",
  "tower",
  "tracing",

zebra-consensus/Cargo.toml

@@ -10,6 +10,7 @@ edition = "2018"
 [dependencies]
 zebra-chain = { path = "../zebra-chain" }
 zebra-state = { path = "../zebra-state" }
+futures-util = "0.3.5"
 tower = "0.3.1"
 
 [dev-dependencies]

zebra-consensus/src/verify.rs

@@ -8,6 +8,7 @@
 //! Verification is provided via a `tower::Service`, to support backpressure and batch
 //! verification.
 
+use futures_util::FutureExt;
 use std::{
     error,
     future::Future,
@@ -80,7 +81,7 @@ where
             block: block.into(),
         });
 
-        Box::pin(async move { Ok(header_hash) })
+        async move { Ok(header_hash) }.boxed()
     }
 }