GH-3320: Refine lifecycle control in StdIntFlow (#3322)

* GH-3320: Refine lifecycle control in StdIntFlow

Fixes https://github.com/spring-projects/spring-integration/issues/3320

Turns out that lifecycle control for the whole bunch of components
in one `IntegrationFlow` is useful in fields.

* Change the logic in the `StandardIntegrationFlow` to let to call
`start()` and `stop()` independently how the flow was registered in
the application context.
This way it can be autowired as a `Lifecycle` to let end-user to
avoid the search for proper component in the flow to stop or start
manually - all the components registered with the flow are going
to be stopped or started respectively

* * Add `this.` to class property usage to satisfy Checkstyle

Co-authored-by: Artem Bilan <abilan@vmware.com>

src/reference/asciidoc/dsl.adoc

@@ -75,6 +75,7 @@ The following list includes the common DSL method names and the associated EIP e
 Conceptually, integration processes are constructed by composing these endpoints into one or more message flows.
 Note that EIP does not formally define the term 'message flow', but it is useful to think of it as a unit of work that uses well known messaging patterns.
 The DSL provides an `IntegrationFlow` component to define a composition of channels and endpoints between them, but now `IntegrationFlow` plays only the configuration role to populate real beans in the application context and is not used at runtime.
+However the bean for `IntegrationFlow` can be autowired as a `Lifecycle` to control `start()` and `stop()` for the whole flow which is delegated to all the Spring Integration components associated with this `IntegrationFlow`.
 The following example uses the `IntegrationFlows` factory to define an `IntegrationFlow` bean by using EIP-methods from `IntegrationFlowBuilder`:
 
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