This patch does various minor style and simplification cleanups, fixing things
detected by tools such as go vet, gofmt -s, and golint.
There are no functional changes, this change is purely cosmetic, but will
enable us to run those tools more regularly now that their output is clean.
This patch introduces the couriers, which the queue uses to deliver mail.
We have a local courier (using procmail), a remote courier (uses SMTP), and a
router courier that decides which of the two to use based on a list of local
domains.
There are still a few things pending, but they all have their basic
functionality working and tested.
