JVMCI code refactoring based on JDK-8220623 reviews.

- Changed MetadataHandleBlock fields which are used only by one instance to static.
- Renamed field _jmetadata::_handle to _value and corresponding access methods because it was confusing: handle->handle().
- Switched from JNIHandleBlock to OopStorage use for _object_handles.
- Additional JVMCI and Graal changes for libgraal from Bob V.