When deciding whether to map String/Array addresses for defragmenting field indexes, defragment looks at the metadata for the class index it currently is processing. This may be a parent class which doesn't know about its child class' indexed fields, so defragment will fail to create mappings for those addresses and the field indexes will be broken.

Proposed fix:
- Read ObjectHeader in SecondPassCommand#processObjectSlot() and use the resulting most specific class metadata to decide whether addresses should be mapped.

Performance improvements:
- Pass Buffer on to ReaderPair#processCopy() so it isn't read twice.
- Cache information on whether to map addresses in a ClassMetadata -> boolean map within the defrag context