Reorder workflow functions to be in the order that they run

Make it a little easier for people who are looking to follow the
workflow we have in our `main()` function by placing the workflow
functions in the other that we run them.
Explicitly log any errors we have trying to acknowledge messages

`Delivery` objects return an error if they can't be acknowledged. Log
when we receive an error so that we know where these errors are
appearing.
[#76261684] Increase queue read channel buffer based on number of crawlers

Increase the buffer we hold for `Delivery` items to equal the number
of goroutines we're running for crawling.
Merge pull request #50 from alphagov/increase-reader-channel-buffer-to-equal-number-of-crawler-threads

[#76261684] Log acknowledgement errors and increase reader channel buffer