https://build.spring.io/browse/INT-MASTERSPRING40-553
The `ImapMailReceiverTests` fails sporadically according some race
condition or wrong logic.
* Fix `ImapIdleChannelAdapter` to check for folder not null before
performing logic in the `IdleTask`
* Polishing for logs which are based on constant strings
* Remove `volatile` from configuration properties in the
`ImapIdleChannelAdapter` and `AbstractMailReceiver`
* Refactor some smells into the `protected` getters instead of direct
access to the property
* Stop channel adapters in the `ImapMailReceiverTests`
* Also destroy task schedulers in the `ImapMailReceiverTests`
33e52486cf