Fix regression on threadpool termination

After the Scala 3 change, cluster.disconnect() stops
the threadpool but no longer blocks on that
termination.

Change-Id: I8e823f2e1e51f92db4b727a324785be4a72a6ebd
Reviewed-on: https://review.couchbase.org/c/couchbase-jvm-clients/+/230811
Reviewed-by: Graham Pople <[email protected]>
Tested-by: Build Bot <[email protected]>

Author: programmatix

scala-client/src/main/scala/com/couchbase/client/scala/env/ClusterEnvironment.scala

@@ -16,7 +16,7 @@
 
 package com.couchbase.client.scala.env
 
-import java.util.concurrent.{Executors, ThreadFactory}
+import java.util.concurrent.{Executors, ThreadFactory, TimeUnit}
 import com.couchbase.client.core
 import com.couchbase.client.core.annotation.SinceCouchbase
 import com.couchbase.client.core.annotation.Stability.{Uncommitted, Volatile}
@@ -401,6 +401,7 @@ class ClusterEnvironment(private[scala] val builder: ClusterEnvironment.Builder)
     if (!threadPool.isShutdown) {
       coreEnv.shutdown(timeout)
       threadPool.shutdownNow()
+      threadPool.awaitTermination(timeout.toMillis, TimeUnit.MILLISECONDS)
       defaultScheduler.dispose()
     }
   }