* Improve ImapIdleChannelAdapter
* We should not destroy a `TaskExecutor` in the `stop()`, especially
when we are going to restart eventually.
Move that logic into `destroy()`
* we should not destroy `MailReceiver` in the `stop()`; we don't
reinstate it in the `start()`.
Move the logic into `destroy()`
* Wrap `ReceivingTask` and `IdleTask` into `isRunning()` condition to
avoid task executions when we are in stopped state
* Remove `ImapMailReceiverTests.testExecShutdown()` since it is not
relevant any more and doesn't reflect `mail` module requirements
* * Add `ImapMailReceiver.cancelPing()` hook
* Also close folder for each `stop()`, as well as in the `destroy()`
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