(#10) - add seq checkpoints

I remember now why I suggested including the `seq` in a separate line - it's just easier given the way replication works, since the checkpoints are written as a `_local` doc after the documents themselves are written.

Two more benefits: 1) potentially we could batch the stream, so that the checkpoints come e.g. every 50 docs. 2) it's clearer whether the seq applies before vs. after the written documents.

So I'm happy with this PR.