JIRA: https://jira.spring.io/browse/INT-4163
When `MessageSource` is proxy, the `TransactionSynchronizationManager.getResource(this)` logic in the `MessageSource` doesn't work,
because TX resource is bound to the `Proxy` in the `SourcePollingChannelAdapter`
* Introduce `SourcePollingChannelAdapter.originalSource` property and store there a target `MessageSource` object extracted from the AOP Proxy
* Use `originalSource` as a resource to bind to the TX
* Modify `MongoDbInboundChannelAdapterIntegrationTests` to ensure that `AbstractMessageSourceAdvice` proxying the `MessageSource` doesn't effect `TransactionSynchronizationManager.getResource(this)` logic
* Refactor for some MongoDb test to rely on the `@RunWith(SpringJUnit4ClassRunner.class)` for context loading for better test class performance
**Cherry-pick to 4.3.x, 4.2.x**
Fallback to provided source if `target` from Proxy is `null`
Fix [UnusedImport] issue
e2d18054e6