It's used in `fetch_existing_msgs()`, but we can remove it and tell users that they need to
move/copy messages from Sentbox to Inbox so that Delta Chat adds all contacts from them. This way
users will be also informed that Delta Chat needs users to CC/BCC/To themselves to see messages sent
from other MUAs.
The motivation is to reduce code complexity, get rid of the extra IMAP connection and cases when
messages are added to chats by Inbox and Sentbox loops in parallel which leads to various message
sorting bugs, particularly to outgoing messages breaking sorting of incoming ones which are fetched
later, but may have a smaller "Date".
We do not try to delete resent messages
anymore. Previously resent messages
were distinguised by having duplicate Message-ID,
but future Date, but now we need to download
the message before we even see the Date.
We now move the message to the destination folder
but do not fetch it.
It may not be a good idea to delete
the duplicate in multi-device setups anyway,
because the device which has a message
may delete the duplicate of a message
the other device missed.
To avoid triggering IMAP busy move loop
described in the comments
we now only move the messages
from INBOX and Spam folders.
