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.
This patch introduces a basic on disk configuration, comprised of a main
configuration file and per-domain directories.
It's still not complete, but will be extended in subsequent patches.
This patch adds some tests that cover the SMTP commands, including STARTTLS
and various correctness checks.
There are also two simple benchmarks, that are not optimized and are more
useful for stress testing and profiling than anything else.
