IDLE mailbox listener leak

What

On a production instance running James 3.7.0 I have 2900 + IDLE listeners for only 58 IMAP connections. Something is clearly wrong here!

See attached screenshot.

A code audit showed that the listener associated with IMAP IDLE is only removed when the use inputs "DONE" to close the IDLE command.

If the connection is unexpectedly dropped then the listener is never removed!

This is even more critical that this keeps the UID-MSN mapping in memory forver .

The fix

This is fixed on master (upcoming 3.8.0) as we no longer put a distinct registration for IMAP IDLE but reuse the connection for the Selected mailbox.

Selected mailbox is cleaned up upon disconnection, deselection etc.. so we don't need to worry.

Also it puts less of a churn on RabbitMQ component as IDLE becomes essentially a hook on the SelectedMailboxImpl listener.

I propose to backport this to 3.7.1...