SyncV2

[ChrisPenner]

TODO:

• Plan for future compatibility as best we can (leave gaps in UCM side so we can include additional metadata without erroring)
• Figure out plans for how UCM can tell Share to omit subtrees/causals
• Maybe mark certain causals as being part of the top-level spine?
• Consider what else is necessary for making this work with push as well.
• More testing with storing dependency depth in the DB and using that for sorting things.

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Overview

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