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)
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