Fix: Netty threads may still linger after client close (apache#5628)

### Motivation

Even after the synchronous close() method is called on a PulsarClient, netty thread(s) from the EventLoopGroup used in the ConnectionPool may still linger around for another 2 seconds afterwards due to not wait for the the graceful shutdown to complete.

While this usually doesn't cause any problems for a normal use case, it does cause problems in the Pulsar Flink sources and sinks.  Exceptions like the following can happen when the Flink job with a Pulsar source or sink gets closed:

```
Exception in thread "pulsar-client-io-1-1" java.lang.NoClassDefFoundError: org/apache/pulsar/shade/io/netty/buffer/PoolArena$1
	at org.apache.pulsar.shade.io.netty.buffer.PoolArena.freeChunk(PoolArena.java:293)
	at org.apache.pulsar.shade.io.netty.buffer.PoolThreadCache$MemoryRegionCache.freeEntry(PoolThreadCache.java:460)
	at org.apache.pulsar.shade.io.netty.buffer.PoolThreadCache$MemoryRegionCache.free(PoolThreadCache.java:430)
	at org.apache.pulsar.shade.io.netty.buffer.PoolThreadCache$MemoryRegionCache.free(PoolThreadCache.java:422)
	at org.apache.pulsar.shade.io.netty.buffer.PoolThreadCache.free(PoolThreadCache.java:279)
	at org.apache.pulsar.shade.io.netty.buffer.PoolThreadCache.free(PoolThreadCache.java:270)
	at org.apache.pulsar.shade.io.netty.buffer.PoolThreadCache.free(PoolThreadCache.java:241)
	at org.apache.pulsar.shade.io.netty.buffer.PooledByteBufAllocator$PoolThreadLocalCache.onRemoval(PooledByteBufAllocator.java:450)
	at org.apache.pulsar.shade.io.netty.buffer.PooledByteBufAllocator$PoolThreadLocalCache.onRemoval(PooledByteBufAllocator.java:426)
	at org.apache.pulsar.shade.io.netty.util.concurrent.FastThreadLocal.remove(FastThreadLocal.java:271)
	at org.apache.pulsar.shade.io.netty.util.concurrent.FastThreadLocal.removeAll(FastThreadLocal.java:67)
	at org.apache.pulsar.shade.io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:32)
	at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.ClassNotFoundException: org.apache.pulsar.shade.io.netty.buffer.PoolArena$1
	at java.net.URLClassLoader.findClass(URLClassLoader.java:382)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
	at org.apache.flink.runtime.execution.librarycache.FlinkUserCodeClassLoaders$ChildFirstClassLoader.loadClass(FlinkUserCodeClassLoaders.java:129)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
	... 13 more
```

This is because threads from the EventLoopGroup lingers after the pulsar client is closed and the Flink job is unloaded.

### Modifications

Shorten the graceful shutdown period and wait for it to complete

pulsar-client/src/main/java/org/apache/pulsar/client/impl/ConnectionPool.java

@@ -40,6 +40,7 @@
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.TimeUnit;
 import java.util.function.Supplier;
 
 import org.apache.pulsar.client.api.PulsarClientException;
@@ -281,8 +282,8 @@ private CompletableFuture<Channel> connectToAddress(InetAddress ipAddress, int p
 
     @Override
     public void close() throws IOException {
-        eventLoopGroup.shutdownGracefully();
         dnsResolver.close();
+        eventLoopGroup.shutdown();
     }
 
     private void cleanupConnection(InetSocketAddress address, int connectionKey,