Mark messages as seen in IMAP loop

MarkseenMsgOnImap job, that was responsible for marking messages as
seen on IMAP and sending MDNs, has been removed.

Messages waiting to be marked as seen are now stored in a
single-column imap_markseen table consisting of foreign keys pointing
to corresponding imap table records.

Messages are marked as seen in batches in the inbox loop. UIDs are
grouped by folders to reduce the number of requests, including folder
selection requests. UID grouping logic has been factored out of
move_delete_messages into UidGrouper iterator to avoid code duplication.

Messages are marked as seen right before fetching from the inbox
folder. This ensures that even if new messages arrive into inbox while
the connection has another folder selected to mark messages there, all
messages are fetched before going IDLE. Ideally marking messages as
seen should be done after fetching and moving, as it is a low-priority
task, but this requires skipping IDLE if UIDNEXT has advanced since
previous time inbox has been selected. This is outside of the scope of
this change.

MDNs are now queued independently of marking the messages as seen.
SendMdn job is created directly rather than after marking the message
as seen on IMAP. Previously sending MDNs was done in MarkseenMsgOnImap
avoid duplicate MDN sending by setting $MDNSent flag together with
\Seen flag and skipping MDN sending if the flag is already set. This
is not the case anymore as $MDNSent flag support has been removed in
9c077c98cd and duplicate MDN sending in
multi-device case is avoided by synchronizing Seen status since
833e5f46cc as long as the server
supports CONDSTORE extension.

src/sql.rs

@@ -181,6 +181,7 @@ impl Sql {
                      PRAGMA secure_delete=on;
                      PRAGMA busy_timeout = {};
                      PRAGMA temp_store=memory; -- Avoid SQLITE_IOERR_GETTEMPPATH errors on Android
+                     PRAGMA foreign_keys=on;
                      ",
                     Duration::from_secs(10).as_millis()
                 ))?;