Use atomic u64 instead of Mutex for counter

This change switches from `Mutex<u64>` to `AtomicU64`, which is faster
and somewhat more elegant to write. It keeps the `Counter` type for
backwards compatibility.

The downside is that it doesn't work on as many platforms and requires
fairly recent compiler.

src/daemon.rs

@@ -13,7 +13,7 @@ use std::collections::{HashMap, HashSet};
 use std::io::{BufRead, BufReader, Lines, Write};
 use std::net::{SocketAddr, TcpStream};
 use std::path::PathBuf;
-use std::sync::{Arc, Mutex};
+use std::sync::{Arc, Mutex, atomic::{AtomicU64, Ordering}};
 use std::time::Duration;
 
 use crate::errors::*;
@@ -267,20 +267,19 @@ impl Connection {
 }
 
 struct Counter {
-    value: Mutex<u64>,
+    value: AtomicU64,
 }
 
 impl Counter {
     fn new() -> Self {
         Counter {
-            value: Mutex::new(0),
+            value: 0.into(),
         }
     }
 
     fn next(&self) -> u64 {
-        let mut value = self.value.lock().unwrap();
-        *value += 1;
-        *value
+        // fetch_add() returns previous value, we want current one
+        self.value.fetch_add(1, Ordering::Relaxed) + 1
     }
 }