JIRA: https://jira.spring.io/browse/INT-4270
Initial Commit
Remove `final` modifier from certain critical framework methods that prevent CGLib proxies working.
Since Spring Boot 2.0 now uses proxyTargetClass=true by default (and has done that for
transactional proxies since 1.4), we must relax these restrictions so that CGLib can
proxy channels (which is often done to make subflows run in a transaction).
When CGLib can't override a `final` method, the fields used within those methods are uninitialized.
In the case of `AbstractMessageChannel` this causes an NPE on the unitiallized `dataTypes` field.
With `MessageHandler`, fields like `shouldTrack` are always false.
Further, methods on `IntegrationObjectSupport` - such as `getComponentName()` return null.
I have left setters and `afterPropertiesSet` (on IOS) `final` since these will typically be called
before the object is proxied.
I suspect there will be other methods we need to open up, but perhaps we should only do them on-demand.
__cherry-pick to 4.3.x__
2b4c343eaa