To avoid `Message` re-creation during `AbstractMessageSource.receive()`
logic, refactor `AbstractMessageSource` implementations
to return `AbstractIntegrationMessageBuilder`
* Add `AbstractIntegrationMessageBuilder<File> doReceive()` to the
`FileReadingMessageSource` to be called from the `AbstractInboundFileSynchronizingMessageSource`
to avoid message recreation in its `doReceive()`
* Some code style refactoring in the `AbstractMessageSource`
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