Summary
-------

Convert the ClassLoader implementation from its use of finalizer to phantom
reference to handle unloading of a native library when a class loader is unloaded.

Problem
-------

The finalization mechanism is inherently problematic and the JDK implementation 
should migrate away from its use of finalizers.

Solution
--------

Use java.lang.ref.Cleaner to register the class loader and the action to unload
a native library.

Specification
-------------

   * `JNI_FindClass`
    
    Specify that `JNI_FindClass` uses the system class loader as its context for finding
    class when `JNI_FindClass` is called from `JNI_OnUnload` or `JNI_OnUnload_L`.

    This is behavioral change because the hotspot implementation has been using
    the class loader associated with the native library as the current context.  It is 
    a bug because the spec of `JNI_OnUnload` specifies that it is called in an 
    unknown context (such as from a finalizer).
<p>

   * `JNI_OnUnload` and `JNI_OnUnload_L`

   Change the spec as follows to reflect that the native library may be unloaded 
   after the class loader is GC'ed.

    <pre>
-Optional function defined by dynamically linked libraries. The VM calls
-`JNI_OnUnload` when the class loader containing the native library is garbage
-collected.
+Optional function defined by dynamically linked libraries.
+When the class loader containing the native library is garbage collected,
+the VM registers `JNI_OnUnload` to be invoked when the native library
+is being unloaded.  It is not specified when the `JNI_OnUnload` function
+is invoked and the native library is unloaded.
    </pre>

    <pre>
-Optional function defined by statically linked libraries. When the class loader
-containing a statically linked native library 'L' is garbage collected, the VM
-will invoke the `JNI_OnUnload_L` function of the library if such a function is
-exported.
+Optional function defined by statically linked libraries.
+When the class loader containing a statically linked native library 'L'
+is garbage collected, the VM registers `JNI_OnUnload_L` to be invoked
+if such a function is exported.  It is not specified when the 
+`JNI_OnUnload_L` function is invoked.

</pre>