Fix HUDI-27 : Support num_cores > 1 for writing through spark

- Users using spark.executor.cores > 1 used to fail due to "FileSystem closed"
 - This is due to HoodieWrapperFileSystem closing the wrapped filesytem obj
 - FileSystem.getInternal caching code races threads and closes the extra fs instance(s)
 - Bumped up num cores in tests to 8, speeds up tests by 3-4 mins

hoodie-client/src/main/java/com/uber/hoodie/io/storage/HoodieWrapperFileSystem.java

@@ -536,8 +536,9 @@ public class HoodieWrapperFileSystem extends FileSystem {
   }
 
   @Override
-  public void close() throws IOException {
-    fileSystem.close();
+  public void close() {
+    // Don't close the underlying `fileSystem` object. This will end up closing it for every thread since it
+    // could be cached across jvm. We don't own that object anyway.
   }
 
   @Override