imapserver: implement NOTIFY extension from RFC 5465

NOTIFY is like IDLE, but where IDLE watches just the selected mailbox, NOTIFY
can watch all mailboxes. With NOTIFY, a client can also ask a server to
immediately return configurable fetch attributes for new messages, e.g. a
message preview, certain header fields, or simply the entire message.

Mild testing with evolution and fairemail.

smtpserver/server.go

@@ -3454,7 +3454,7 @@ func (c *conn) deliver(ctx context.Context, recvHdrFor func(string) string, msgW
 						if err := tx.Update(mbrej); err != nil {
 							return fmt.Errorf("updating rejects mailbox: %v", err)
 						}
-						changes = append(changes, a.d.m.ChangeAddUID(), mbrej.ChangeCounts())
+						changes = append(changes, a.d.m.ChangeAddUID(*mbrej), mbrej.ChangeCounts())
 						stored = true
 						return nil
 					})

smtpserver/server_test.go

@@ -624,7 +624,8 @@ func TestDelivery(t *testing.T) {
 
 			changes := make(chan []store.Change)
 			go func() {
-				changes <- ts.comm.Get()
+				_, l := ts.comm.Get()
+				changes <- l
 			}()
 
 			timer := time.NewTimer(time.Second)