Fix IMAP race condition around idle()

`IMAPFolder.idle()` by default keeps idle-ing after each response.
We don't need that, because we want to fetch the new mails immediately (over the same connection).
To make `idle()` not keep on going, we called
`folder.isOpen()` which in most cases makes `idle()` stop by calling `noop()`
internally (to keep the connection alive) which leads to the current
`idle()`-call being canceled.

The problem this commits addresses is that the call to `noop()` only
happens if there were no calls in the last second.
There is a check for that in `com.sun.mail.imap.IMAPFolder.keepConnectionAlive`.
So if a new message appears less then a second after idle started, we will miss it.

This new way interrupts `idle()` more often than before, e.g. when there
is an expunge-message (i.e. a message was deleted), which we don't care about.
But the subsequent `receive()` in IdleTask will simply get no
messages and then turn around and start idle-ing again.

**Cherry-pick to `5.3.x` & `master`**

spring-integration-mail/src/test/java/org/springframework/integration/mail/ImapMailReceiverTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -91,6 +91,7 @@ import com.sun.mail.imap.IMAPFolder;
  * @author Oleg Zhurakousky
  * @author Gary Russell
  * @author Artem Bilan
+ * @author Alexander Pinske
  */
 @SpringJUnitConfig
 @ContextConfiguration(
@@ -607,7 +608,7 @@ public class ImapMailReceiverTests {
 			Thread.sleep(300);
 			shouldFindMessagesCounter.set(1);
 			return null;
-		}).given(folder).idle();
+		}).given(folder).idle(true);
 
 		adapter.start();
 
@@ -667,7 +668,7 @@ public class ImapMailReceiverTests {
 			idles.countDown();
 			Thread.sleep(500);
 			return null;
-		}).given(folder).idle();
+		}).given(folder).idle(true);
 
 		adapter.start();