fix: unref setTimeout in pool

As a library don't block the process quit. Simply `unref` all the `setTimeout`s.

lib/Pool.js

@@ -402,7 +402,7 @@ class Pool extends EventEmitter {
       this._scheduledEviction = setTimeout(() => {
         this._evict();
         this._scheduleEvictorRun();
-      }, this._config.evictionRunIntervalMillis);
+      }, this._config.evictionRunIntervalMillis).unref();
     }
   }