Reset timer instead of start() when stepDown (sofastack#408)

* Fix

* Refine

* address comment

* Fix

jraft-core/src/main/java/com/alipay/sofa/jraft/core/NodeImpl.java

@@ -1266,7 +1266,7 @@ private void stepDown(final long term, final boolean wakeupCandidate, final Stat
         }
         // Learner node will not trigger the election timer.
         if (!isLearner()) {
-            this.electionTimer.start();
+            this.electionTimer.restart();
         } else {
             LOG.info("Node {} is a learner, election timer is not started.", this.nodeId);
         }

jraft-core/src/main/java/com/alipay/sofa/jraft/util/RepeatedTimer.java

@@ -151,6 +151,28 @@ public void start() {
         }
     }
 
+    /**
+     * Restart the timer.
+     * It will be started if it's stopped, and it will be restarted if it's running.
+     *
+     * @author Qing Wang ([email protected])
+     *
+     * 2020-Mar-26 20:38:37 PM
+     */
+    public void restart() {
+        this.lock.lock();
+        try {
+            if (this.destroyed) {
+                return;
+            }
+            this.stopped = false;
+            this.running = true;
+            schedule();
+        } finally {
+            this.lock.unlock();
+        }
+    }
+
     private void schedule() {
         if (this.timeout != null) {
             this.timeout.cancel();