Breaking suggestion: add support for transactions

MongoDB 3.x was end-of-lifed in April 2021, which means that all currently-supported versions of MongoDB now support transactions.

sharedb-mongo currently uses an optimistic write mechanism to deal with the fact that it has to write both an op and a snapshot as part of a single "commit". This results in some non-canonical ops being committed to the database, and means we need to do some painful work to get a range of canonical ops.

If we moved to using MongoDB transactions, we would know that all committed ops are canonical (since a conflicting transaction can be made to roll back), which means we could vastly simplify op range fetches.

This would be a breaking change, since it would require MongoDB >= 4.X.

We'd also want to do some thinking about how to enable this feature on already-established databases. For example, if you turn on transactions in sharedb-mongo, any documents that were created after the change can be considered to only have canonical ops, but any "legacy" documents created before that time would not. We could potentially set a flag on the snapshots?

Start by tracing sharedb-mongo’s current optimistic write mechanism for committing an op and snapshot, then review MongoDB transactions and the existing canonical-op range-fetch behavior. Define how transactions would replace the current mechanism, how legacy documents would be identified, and how enabling the change would affect existing databases.