Support ChangeSet merging and serialization

For pre-filtering for external changes we would need support in ChangeSet for

1. Merging multiple ChangeSet into single -
   1. This would be used to build up ChangeSet from each commit which would be used in Background Write in DocumentNodeStore
   2. And again while building ChangeSet from multiple JournalEntries while doing Background Read
   3. Care must be taken for overflow handling while merging
2. Serializing and Deserializing ChangeSet as JSON - Care must be taken to distinguish between an empty attribute vs overflown attribute