SNOW-569611 do not allow concurrent client.close()

src/main/java/net/snowflake/ingest/streaming/internal/SnowflakeStreamingIngestClientInternal.java

@@ -528,7 +528,7 @@ List<BlobMetadata> getRetryBlobs(
 
   /** Close the client, which will flush first and then release all the resources */
   @Override
-  public void close() throws Exception {
+  public synchronized void close() throws Exception {
     if (isClosed) {
       return;
     }