refactor(lib): Import tracing macros per-module

Instead of one #[macro_use] at the crate root.

src/server/shutdown.rs

@@ -2,6 +2,7 @@ use std::error::Error as StdError;
 
 use pin_project_lite::pin_project;
 use tokio::io::{AsyncRead, AsyncWrite};
+use tracing::debug;
 
 use super::accept::Accept;
 use super::conn::{SpawnAll, UpgradeableConnection, Watcher};