This patch:

1. Copies unsent messages to the outbox before attempting to deliver
   them with SMTP
2. Deletes those messages once they're sent, or leaves them if an error
   occured
3. Updates the message list to make it obvious when there are unsent
   messages in the outbox

This one is a bit of a doozy. A summary of the changes:

- Session has grown storage for attachments which have been uploaded but
  not yet sent.
- The list of attachments on a message is refcounted so that we can
  clean up the temporary files only after it's done with - i.e. after
  copying to Sent and after all of the SMTP attempts are done.
- Abandoned attachments are cleared out on process shutdown.

Future work:
- Add a limit to the maximum number of pending attachments the user can
  have in the session.
- Periodically clean out abandoned attachments?

References: https://todo.sr.ht/~sircmpwn/koushin/16

Note that attachments will be lost. This is a TODO.

Closes: https://todo.sr.ht/~sircmpwn/koushin/13

This plugin offers base IMAP/SMTP functionality.

References: https://todo.sr.ht/~sircmpwn/koushin/39

References: https://todo.sr.ht/~sircmpwn/koushin/22

HTTP requests can be processed in parallel, but we only have one IMAP
connection per session.

Closes: https://todo.sr.ht/~sircmpwn/koushin/12