Fix a slow gc thread shutdown when compacting (apache#4127)

Descriptions of the changes in this PR:

### Motivation

Fix a slow gc thread shutdown when compacting.

The problem here is that the stop flag `running` has been moved after `compacting.compareAndSet`. When `running` is not set to false, entry log continues to compact one after one and the shutdown thread is hard to set the `compacting`.

### Changes

Set `running` to false first and then check `compacting`.

Master Issue: apache#4126

bookkeeper-server/src/main/java/org/apache/bookkeeper/bookie/GarbageCollectorThread.java

@@ -679,6 +679,7 @@ public synchronized void shutdown() throws InterruptedException {
         if (!this.running) {
             return;
         }
+        this.running = false;
         LOG.info("Shutting down GarbageCollectorThread");
 
         throttler.cancelledAcquire();
@@ -688,7 +689,6 @@ public synchronized void shutdown() throws InterruptedException {
             Thread.sleep(100);
         }
 
-        this.running = false;
         // Interrupt GC executor thread
         gcExecutor.shutdownNow();
         try {