使用线程池管理线程，新增worker_threads参数

src/configuration.rs

@@ -14,6 +14,7 @@ pub struct Configuration {
     spam_file: String,
     spam_delay_ms: i32,
     random_prefix: bool,
+    pub worker_threads: i32
 }
 
 impl Configuration {
@@ -56,6 +57,7 @@ impl Configuration {
             spam_file: "".to_string(),
             spam_delay_ms: 0,
             random_prefix: false,
+            worker_threads: 100
         }
     }