Continue to overwrite the bill as long as it's Estimated

[kevinburke]

Currently we mark a bill as "finalized" when a new month's bill
appears in S3, but AWS continues to update a bill after a month is
over for several days. The correct way to check for a "finalized" bill
is to see whether the invoice_id row has a number or is marked as
"Estimated". As long as the bill is estimated, we want to overwrite it
with new data.

We can't do this earlier on because that would require peeking at the
S3 data, so overwrite the data in the line_items table as long as it's
not a finalized bill.

Finally, some rows appear in the spreadsheet for a given month as
"totals" of the other rows. These rows do not have an invoice_id (it's
empty) hence the check for '' below.

[kevinburke]

I just pushed an additional change. Previously we'd cache a stale staged DBR, when Amazon updated the DBR in the original bucket we'd still use the stale one from the staging bucket for the line_items table. Fix this by fetching the stage and the original bucket simultaneously, and only using the stage CSV if it has the same modified date as the CSV in the original bucket.

Again this is an indication that if you are using this project as-is you are probably missing records and updates from Amazon between the end of a month and the time that Amazon sends you an invoice, which can be several days later.

[kevinburke]

(This work was sponsored by Segment)