[Improvement] Shutdown the grpc executors pool when closing (#83)

### What changes were proposed in this pull request?
[Improvement] Shutdown the grpc executors pool when closing

### Why are the changes needed?
Although the executor pool is used in grpc, but the grpc server won't take ownership of the given executor. It's caller's responsibility to shut down the executor when it's desired.

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
No need.

common/src/main/java/org/apache/uniffle/common/rpc/GrpcServer.java

@@ -41,12 +41,13 @@ public class GrpcServer implements ServerInterface {
 
   private final Server server;
   private final int port;
+  private final ExecutorService pool;
 
   public GrpcServer(RssBaseConf conf, BindableService service, GRPCMetrics grpcMetrics) {
     this.port = conf.getInteger(RssBaseConf.RPC_SERVER_PORT);
     long maxInboundMessageSize = conf.getLong(RssBaseConf.RPC_MESSAGE_MAX_SIZE);
     int rpcExecutorSize = conf.getInteger(RssBaseConf.RPC_EXECUTOR_SIZE);
-    ExecutorService pool = new ThreadPoolExecutor(
+    pool = new ThreadPoolExecutor(
         rpcExecutorSize,
         rpcExecutorSize * 2,
         10,
@@ -89,6 +90,9 @@ public void stop() throws InterruptedException {
       server.shutdown().awaitTermination(10, TimeUnit.SECONDS);
       LOG.info("GRPC server stopped!");
     }
+    if (pool != null) {
+      pool.shutdown();
+    }
   }
 
   public void blockUntilShutdown() throws InterruptedException {