[KYUUBI #6790] Fix engine cannot exit when gracefully stopped

# 🔍 Description
## Issue References 🔗

Fix engine cannot exit when gracefully stopped

This pull request fixes #6790

## Describe Your Solution 🔧

## Types of changes 🔖

- [X] Bugfix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing functionality to change)

## Test Plan 🧪

#### Behavior Without This Pull Request ⚰️

#### Behavior With This Pull Request 🎉

#### Related Unit Tests

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# Checklist 📝

- [X] This patch was not authored or co-authored using [Generative Tooling](https://www.apache.org/legal/generative-tooling.html)

**Be nice. Be informative.**

Closes #6792 from wForget/KYUUBI-6790.

Closes #6790

efe7a2f [wforget] [KYUUBI-6790] Fix engine cannot exit when gracefully stopped

Authored-by: wforget <[email protected]>
Signed-off-by: Wang, Fei <[email protected]>
(cherry picked from commit b4838b4)
Signed-off-by: Wang, Fei <[email protected]>

Author: wForget

kyuubi-ha/src/main/scala/org/apache/kyuubi/ha/client/ServiceDiscovery.scala

@@ -66,10 +66,11 @@ abstract class ServiceDiscovery(
 
   // stop the server genteelly
   def stopGracefully(isLost: Boolean = false): Unit = {
-    val activeSessionCount = fe.be.sessionManager.getActiveUserSessionCount
+    var activeSessionCount = fe.be.sessionManager.getActiveUserSessionCount
     while (activeSessionCount > 0) {
       info(s"$activeSessionCount connection(s) are active, delay shutdown")
       Thread.sleep(TimeUnit.SECONDS.toMillis(10))
+      activeSessionCount = fe.be.sessionManager.getActiveUserSessionCount
     }
     isServerLost.set(isLost)
     gracefulShutdownLatch.countDown()