GH-3451: Use GreenMail for testing instead

Fixes https://github.com/spring-projects/spring-integration/issues/3451

The GreenMail provides a comprehensive support for e-mail scenarios
testing.

* Use GreenMail instead our own `TestMailServer` for better and broader
support
* Convert SmtpTests and Pop3Tests to GreenMail
* Document that Pop3MailReceiver does not support autoCloseFolder=false
* Pop3MailReceiver uses `folder.getMessageCount()` to determine if there
are new messages.
This count only gets updated when opening the folder.
* Use GreenMail for remaining mail tests
* convert MailTests and ImapMailReceiverTests to GreenMail
* remove duplicate mail dependencies
* route com.sun.mail logging to log4j (via jul and slf4j)

spring-integration-test-support/src/main/java/org/springframework/integration/test/mail/TestMailServer.java

@@ -48,7 +48,10 @@ import org.springframework.util.Base64Utils;
  *
  * @since 5.0
  *
+ * @deprecated since 5.5 in favor of GreenMail library for mail testing.
+ *
  */
+@Deprecated
 public final class TestMailServer {
 
 	public static SmtpServer smtp(int port) {