JIRA: https://jira.spring.io/browse/INT-4413
When we override entity in the `IntegrationFlowContext.registry`,
we may get dangling beans in the application context,
when the structure of a new `IntegrationFlow` is different.
* Fix `IntegrationFlowContext` to disallow to override the flow
registration under the same name
* Add JavaDocs to the `IntegrationFlowRegistrationBuilder` methods
* Add `toString()` to the `IntegrationFlowRegistration` and
`StandardIntegrationFlow` to make the logging of these components
friendlier
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