Implement mediachain syndication interface

[parkan]

pending https://github.com/mediachain/mediachain/issues/24

We can do a preliminary implementation already because we generally know what the shape of the interface will be like.

• [ ] JournalEntry to internal Blob mapping (we may want to sync up the types)
• [ ] accept JournalEntry and write corresponding Blob
• [ ] keep track of last known chain head, sanity check incoming updates, error handling
• [ ] [DEFERRED] catchup protocol