Support receiving and sending MIME messages through /mxdeliv endpoint

[j-g00da]

• implement SMTP transport channel
• implement HTTP transport channel
• add nginx rule for /mxdeliv/
• add http server for filtermail-incoming
• keep smtp and http connections alive instead of reestablishing connection on each message.

[j-g00da]

After some discussion, the proper way of doing this would probably be creating a separate binary that is called by a postfix pipe daemon and set it as a default_transport. I'm not sure yet how to configure postfix in a way that first tries pipe and upon failure retries using smtp transport.

On the receiving side we can probably just create a service that listens on a http endpoint and sends whatever comes in to smtpd.

[j-g00da]

For outbound we will probably have to implement both http and smtp delivery channels as a single transport.

For inbound, a simpler alternative would be to listen on http in filtermail-incoming and inject messages directly (skipping submission smtpd). The side effect would be that messages coming through http would skip cleanup service completely.*

*Edit: that's of course not true, this approach should be fine

[link2xt]

pipe spawns a new process for each mail, so maybe it is better to have a long-running SMTP server and configure it as a relay for all messages with default_transport = smtp:127.0.0.1:xxxx. This way the process can maintain outgoing HTTP and SMTP connections and not have to establish a new connection each time.