Fixes spring-cloud/spring-cloud-stream#983 When the `AbstractMessageChannel.getFullChannelName()` is called before the full bean initialization, we don't have the proper name any more lately - that is only `unknown.channel.name` * Reset `fullChannelName` property to `null` in the end of `onInit()`, so the next `getFullChannelName()` will build the proper component name **Cherry-pick to 4.3.x**