Implement weak reference processing and finalisation in mmtk-openjdk

[wks]

Parent issue: mmtk/mmtk-core#694

Previously, mmtk-core includes an implementation of weak reference processor and a finaliser processor from JikesRVM. mmtk-openjdk, the OpenJDK binding, also used that framework. That approach is problematic. (See mmtk/mmtk-core#694)

With the generalised language-neutral weak reference processing API introduced in mmtk-core (See: mmtk/mmtk-core#700), we should implement the reference/finalisation processors in mmtk-openjdk in an OpenJDK-specific manner.

Related work:

1. https://github.com/wks/mmtk-openjdk/tree/gen-weakref-api
   It was my experiment. I copied and pasted the ReferenceProcessor and FinalizableProcessor to mmtk-openjdk with minimal modification, and use the new API to communicate with mmtk-core. This shows the new API works, but the ReferenceProcessor and FinalizableProcessor are still implemented in the JikesRVM style.
2. https://github.com/wenyuzhao/mmtk-openjdk/tree/lxr
   This branch is mainly for the LXR GC algorithm, but it also includes a new implementation of reference/finalisation processor that is more closely integrated with OpenJDK. However, this implementation depends on a modified version of mmtk-core that contains an extended but still Java-specific weak reference processing API.

We should implement reference/finalisation processing in mmtk-openjdk in a way closely related to OpenJDK's own reference/finalisation processing mechanism, but we should use the new language-neutral API.

Key to using the new API is handling the multiple strengths of references. By using the return value of the Scanning::process_weak_refs function, the VM binding can build a state machine to handle soft/weak/final/phantom references while expanding the transitive closure multiple times. (See: https://github.com/wks/mmtk-openjdk/blob/gen-weakref-api/mmtk/src/weak_processor/mod.rs)