6d523e2a70
https://build.spring.io/browse/INT-FATS5IC-685/
Even if we stop `SourcePollingChannelAdapter`, the task on the fly is
still going to be executed.
This way we may have extra calls to our logic.
* Use `CopyOnWriteArrayList` for the `sessionSources` to avoid
`ConcurrentModificationException` during this collection read in the
tests
**Cherry-pick to 5.0.x**
(cherry picked from commit 76c670075b)