JNI: Improve varlen datatype handling in H5A/H5D read/write functions (#2156)

* Improve JNI VL datatype read/write to handle complex combinations

* Implement VL of VL JNI writes and reads

* Add Java VLofVL test for attributtes

* Changes to address review issues

* Fix H5Aread vl blocks

java/test/testfiles/JUnit-TestH5A.txt

@@ -11,6 +11,7 @@ JUnit version 4.11
 .testH5Aget_info_by_name
 .testH5Aget_create_plist
 .testH5Adelete_by_name
+.testH5AVLwrVL
 .testH5Aopen_by_name
 .testH5Aget_info
 .testH5Aget_name
@@ -31,5 +32,5 @@ JUnit version 4.11
 
 Time:  XXXX
 
-OK (29 tests)
+OK (30 tests)