feat: move the messages only from INBOX and Spam folders

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.

src/imap/session.rs

@@ -5,9 +5,11 @@ use anyhow::{Context as _, Result};
 use async_imap::Session as ImapSession;
 use async_imap::types::Mailbox;
 use futures::TryStreamExt;
+use tokio::sync::Mutex;
 
 use crate::imap::capabilities::Capabilities;
 use crate::net::session::SessionStream;
+use crate::tools;
 
 /// Prefetch:
 /// - Message-ID to check if we already have the message.
@@ -40,6 +42,8 @@ pub(crate) struct Session {
 
     pub selected_folder_needs_expunge: bool,
 
+    pub(crate) last_full_folder_scan: Mutex<Option<tools::Time>>,
+
     /// True if currently selected folder has new messages.
     ///
     /// Should be false if no folder is currently selected.
@@ -71,6 +75,7 @@ impl Session {
             selected_folder: None,
             selected_mailbox: None,
             selected_folder_needs_expunge: false,
+            last_full_folder_scan: Mutex::new(None),
             new_mail: false,
         }
     }