Add support for CacheRequestHandlerAdvice

[artembilan]

• Fix AbstractMessageProcessingTransformer to react for the
  AbstractIntegrationMessageBuilder invocation result and don't wrap it
  into the Message
• Demonstrate functionality in the CacheRequestHandlerAdviceTests

[artembilan]

We may reconsider priority for this feature.
There might be some obstacles doing it existing CacheInterceptor way: https://stackoverflow.com/questions/55073263/ehcache-throwing-notserializableexception-when-moving-to-spring-boot-2

[artembilan]

If this is OK, I'll go ahead with docs.

Thanks

[garyrussell]

LGTM

[artembilan]

Pushed Docs change.

[garyrussell]

$ ./gradlew clean testAll --parallel
Starting a Gradle Daemon (subsequent builds will be faster)

> Task :spring-integration-core:javadoc
/Users/grussell/Development/spring-integration/spring-integration-core/src/main/java/org/springframework/integration/dsl/HeaderEnricherSpec.java:691: warning: empty <p> tag
         * <p><ul>
           ^
/Users/grussell/Development/spring-integration/spring-integration-core/src/main/java/org/springframework/integration/dsl/HeaderEnricherSpec.java:712: warning: empty <p> tag
         * <p><ul>
           ^
/Users/grussell/Development/spring-integration/spring-integration-core/src/main/java/org/springframework/integration/handler/advice/CacheRequestHandlerAdvice.java:133: warning: no @param for errorHandler
        public void setErrorHandler(CacheErrorHandler errorHandler) {
                    ^

[artembilan]

@garyrussell ,

I think this is the last issue which can make it into our M1: I have just upgraded to just released Spring Security M2.

Thanks

[garyrussell]

Merged as 735e82e with minor doc polishing.