* INT-4381: MessageSources refactoring
JIRA: https://jira.spring.io/browse/INT-4381
* Make all the out-of-the-box `MessageSource` implementations based
on the `AbstractMessageSource`
* Fix `JdbcPollingChannelAdapterIntegrationTests` for sporadic failure
because of `fixed-rate` for the poller
* Fix HeaderEnricherTests race condition
The `errorChannel()` expect an error in the `testErrorChannel` after
`1000` ms, but at the same time the `poller` in configured for the
`3000` ms.
* Increase all the timeouts for replies
* Decrease a `fixed-delay` on the `poller`
* Some other code style polishing for the `HeaderEnricherTests`
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