Add support for branchable collections

[AndrewSisley]

There is some high level info in an almanac doc here: https://source.almanac.io/folders/database-team-v1-q4qgzK/branchable-collections-Z0SmQmyPcgaYimjMlHuLZPEGM1s38Ewg?mention_id=4192&docView=Commenting

WIP thoughts:

• We do not need a composite for each document change, instead we only need one for each transaction commit. This would reduce the cost (performance, size on disk) of a branchable collection considerably in a lot of use cases. Some event based code may need to change for this. The composite commit would contain the cids of all the changes to the collection within the transaction, instead of essentially one cid per composite.
  • It might be best to submit a txn commited event, and include txn.id on update events instead of aggregate update events into a set before publishing them.
  • Make sure the branchable collection is modified within the commit, not after it is commited - may need before/after on-success operations?.
• We can allow users to query branchable collections at a specific version like we do with documents (the cid query param). This does not however need to be implemented in the initial PR, is just something to bear in mind for now.