MailboxOutgoingSession doesn't mark messages/acks as sent. Instead it records the message IDs via a DeferredSendHandler so they can be marked as sent later, when the file has been uploaded to the mailbox.
MailboxOutgoingSession monitors the amount of data it's sending and stops when it reaches a predetermined limit. This allows mailboxes to enforce a maximum file size for uploads (briar-mailbox#105 (closed)), which in turn helps to prevent DoS attacks by misbehaving contacts.
Closes #2296 (closed) (not #2226 (closed) as the branch name suggests)