Eliminate finalizers in the core codebase

The Object.finalize() method was deprecated in Java 9 and use of PhantomReference instead is recommended for cases where simply relying on code to invoke a close() method is insufficient. So, let's get rid of the 3 finalizers in the WF Core codebase.

Use the PhantomReference utility stuff in wildfly-common. That lib is already a dependency of the code that has the various finalize() impls.