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.
This patch introduces a basic, in-memory queue that only holds emails for now.
This slows down the benchmarks because we don't yet have a way to wait for
delivery (even if fake), that will come in later patches.
