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_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)