Eliminate double-execution of shutdown code.

src/main/java/com/zaxxer/hikari/pool/HikariPool.java

@@ -224,18 +224,21 @@ public final class HikariPool implements HikariPoolMBean, IBagStateListener
 
     public void shutdown()
     {
-        isShutdown = true;
-        houseKeepingTimer.cancel();
-        addConnectionExecutor.shutdown();
+        if (!isShutdown)
+        {
+            isShutdown = true;
+            houseKeepingTimer.cancel();
+            addConnectionExecutor.shutdown();
     
-        LOGGER.info("HikariCP pool {} is being shutdown.", configuration.getPoolName());
-        logPoolState("State at shutdown ");
+            LOGGER.info("HikariCP pool {} is being shutdown.", configuration.getPoolName());
+            logPoolState("State at shutdown ");
     
-        closeIdleConnections();
+            closeIdleConnections();
     
-        if (isRegisteredMbeans)
-        {
-            HikariMBeanElf.unregisterMBeans(configuration, this);
+            if (isRegisteredMbeans)
+            {
+                HikariMBeanElf.unregisterMBeans(configuration, this);
+            }
         }
     }