Add imap table to keep track of message UIDs

`imap` table maps Message-IDs to UIDs on the server. `dc_receive_imf`
no longer gets the UID of the message as an argument and does not
insert the folder and UID of the message into the `msgs`
table. `server_folder` and `server_uid` columns in `msgs` table are
deprecated.

MoveMsg and DeleteMsgOnImap jobs are removed. Now messages are moved
and deleted only in the `fetch_move_delete` procedure that consults
the `target` column of the `imap` table to determine where the message
should go.

Where the message should go is determined after prefetching by the
`imap::target_folder()` procedure.  Messages are only downloaded once
they reach their target folder to avoid race conditions in multidevice
setting, such as:

1. One device trying to FETCH the message while the other tries to
MOVE it.

2. One device marking the message as \Seen in the Inbox while the
other has already copied unseen message to the Movebox and is going to
delete the \Seen message in the Inbox.

3. Device downloads the message from the Inbox while there are newer
messages in the Movebox placed there by the other device, thus
processing the messages out of order.

src/download.rs

@@ -129,24 +129,48 @@ impl Job {
         }
 
         let msg = job_try!(Message::load_from_db(context, MsgId::new(self.foreign_id)).await);
-        let server_folder = msg.server_folder.unwrap_or_default();
-        match imap
-            .fetch_single_msg(context, &server_folder, msg.server_uid)
-            .await
-        {
-            ImapActionResult::RetryLater | ImapActionResult::Failed => {
-                job_try!(
-                    msg.id
-                        .update_download_state(context, DownloadState::Failure)
-                        .await
-                );
-                Status::Finished(Err(anyhow!("Call download_full() again to try over.")))
-            }
-            ImapActionResult::Success | ImapActionResult::AlreadyDone => {
-                // update_download_state() not needed as receive_imf() already
-                // set the state and emitted the event.
-                Status::Finished(Ok(()))
+        let row = job_try!(
+            context
+                .sql
+                .query_row_optional(
+                    "SELECT uid, folder FROM imap WHERE rfc724_mid=? AND target!=''",
+                    paramsv![msg.rfc724_mid],
+                    |row| {
+                        let server_uid: u32 = row.get(0)?;
+                        let server_folder: String = row.get(1)?;
+                        Ok((server_uid, server_folder))
+                    }
+                )
+                .await
+        );
+
+        if let Some((server_uid, server_folder)) = row {
+            match imap
+                .fetch_single_msg(context, &server_folder, server_uid)
+                .await
+            {
+                ImapActionResult::RetryLater | ImapActionResult::Failed => {
+                    job_try!(
+                        msg.id
+                            .update_download_state(context, DownloadState::Failure)
+                            .await
+                    );
+                    Status::Finished(Err(anyhow!("Call download_full() again to try over.")))
+                }
+                ImapActionResult::Success => {
+                    // update_download_state() not needed as receive_imf() already
+                    // set the state and emitted the event.
+                    Status::Finished(Ok(()))
+                }
             }
+        } else {
+            // No IMAP record found, we don't know the UID and folder.
+            job_try!(
+                msg.id
+                    .update_download_state(context, DownloadState::Failure)
+                    .await
+            );
+            Status::Finished(Err(anyhow!("Call download_full() again to try over.")))
         }
     }
 }
@@ -172,14 +196,14 @@ impl Imap {
         // we are connected, and the folder is selected
         info!(context, "Downloading message {}/{} fully...", folder, uid);
 
-        let (_, error_cnt, _) = self
+        let (last_uid, _received) = self
             .fetch_many_msgs(context, folder, vec![uid], false, false)
             .await;
-        if error_cnt > 0 {
-            return ImapActionResult::Failed;
+        if last_uid.is_none() {
+            ImapActionResult::Failed
+        } else {
+            ImapActionResult::Success
         }
-
-        ImapActionResult::Success
     }
 }
 
@@ -309,16 +333,7 @@ mod tests {
              Date: Sun, 22 Mar 2020 22:37:57 +0000\
              Content-Type: text/plain";
 
-        dc_receive_imf_inner(
-            &t,
-            header.as_bytes(),
-            "INBOX",
-            1,
-            false,
-            Some(100000),
-            false,
-        )
-        .await?;
+        dc_receive_imf_inner(&t, header.as_bytes(), "INBOX", false, Some(100000), false).await?;
         let msg = t.get_last_msg().await;
         assert_eq!(msg.download_state(), DownloadState::Available);
         assert_eq!(msg.get_subject(), "foo");
@@ -331,7 +346,6 @@ mod tests {
             &t,
             format!("{}\n\n100k text...", header).as_bytes(),
             "INBOX",
-            1,
             false,
             None,
             false,
@@ -367,7 +381,6 @@ mod tests {
                     Date: Sun, 14 Nov 2021 00:10:00 +0000\
                     Content-Type: text/plain",
             "INBOX",
-            1,
             false,
             Some(100000),
             false,