JIRA: https://jira.spring.io/browse/INT-4132
The race condition is present when polling task may be ran before `MessageSource<?>` has been started.
* Swap the order of `start()` in the `SourcePollingChannelAdapter`.
Since proxying is now applied only for the `MessageSource.receive()` it doesn't hurt to start it before actual proxying.
Just because it is really should be started before performing its `receive()`
* Prove the proper order with the mock test and protect ourselves for the future similar changes
* Also swap the `stop()` order in the `SourcePollingChannelAdapter.
We have to stop/cancel the polling task before discarding internal `MessageSource` lifecycle.
For example with the current state we may close an underlying resource already, but still have the last polling tick.
That may cause any unexpected behaviour
**Cherry-pick to 4.3.x**
Polishing
c31a96d4cb