Remove statistics file when stop.

Worker.php

@@ -1214,6 +1214,10 @@ class Worker
                 posix_kill($worker_pid, SIGINT);
                 Timer::add(self::KILL_WORKER_TIMER_TIME, 'posix_kill', array($worker_pid, SIGKILL), false);
             }
+            // Remove statistics file.
+            if (is_file(self::$_statisticsFile)) {
+                @unlink(self::$_statisticsFile);
+            }
         } // For child processes.
         else {
             // Execute exit.