Use repo_id instead of [repo, org] as partition key and index when possible

[ericboucher]

Thanks to the changes introduced by #112, we now have a more reliable column to use for children streams.

Note, this is potentially a breaking changes and might change the DB index structure.

[ericboucher]

Similarly, use user_id instead os username whenever possible

[ericboucher]

It could be interesting to link this to #95 (Consider running tap output into a target in CI) and catch when we introduce breaking changes by changing partition keys, primary keys and such

[aaronsteers]

@ericboucher - Can we bump the major version when this comes through? I imagine many (most?) usages will break with the new primary key.

Since this is still a (relatively) new tap, I think now is a good time to make the change. I don't know how many dependencies exist on this repo but I think not a ton as of yet. 😄