INT-4488: Require @EnablePublisher

JIRA: https://jira.spring.io/browse/INT-4488

Since `PublisherAnnotationBeanPostProcessor` is quite expensive
(1.5x the CPU used by `MessagingAnnotationPostProcessor` in a simple
application)

* So not register a `PublisherAnnotationBeanPostProcessor` by default.
The `@EnablePublisher` has to be presented on the `@Configuration`
to allow the `@Publisher` AOP
* Change the `<int:annotation-config>` to require a new
`<int:enable-publisher>` sub-element for similar purpose - do not
register `PublisherAnnotationBeanPostProcessor` by default

src/reference/asciidoc/message-publishing.adoc

@@ -15,6 +15,8 @@ Spring Integration provides two approaches: XML and Annotation-driven.
 ==== Annotation-driven approach via @Publisher annotation
 
 The annotation-driven approach allows you to annotate any method with the `@Publisher` annotation, specifying a 'channel' attribute.
+Starting with _version 5.1_, to switch this functionality on, the `@EnablePublisher` annotation must be provided on some `@Configuration` class.
+See <<configuration-enable-integration>> for more information.
 The Message will be constructed from the return value of the method invocation and sent to a channel specified by the 'channel' attribute.
 To further manage message structure, you can also use a combination of both `@Payload` and `@Header` annotations.
 
@@ -105,10 +107,24 @@ As with most other annotation-driven features in Spring, you will need to regist
 You can instead use namespace support for a more concise configuration:
 [source,xml]
 ----
-<int:annotation-config default-publisher-channel="defaultChannel"/>
+<int:annotation-config>
+    <int:enable-publisher default-publisher-channel="defaultChannel"/>
+</int:annotation-config>
 ----
 
-Similar to other Spring annotations (@Component, @Scheduled, etc.), `@Publisher` can also be used as a meta-annotation.
+For Java & Annotation Spring configuration, the `@EnablePublisher` annotation must be used:
+
+[source,java]
+----
+@Configuration
+@EnableIntegration
+@EnablePublisher("defaultChannel")
+public class IntegrationConfiguration {
+    ...
+}
+----
+
+Similar to other Spring annotations (`@Component`, `@Scheduled`, etc.), `@Publisher` can also be used as a meta-annotation.
 That means you can define your own annotations that will be treated in the same way as the `@Publisher` itself.
 
 [source,java]

src/reference/asciidoc/whats-new.adoc

@@ -51,7 +51,20 @@ Starting with _version 5.0.5_, generated bean names for the components in an `In
 
 See <<java-dsl-flows>> for more information.
 
-==== AMQP Changes
+==== Aggregator Changes
+
+An aggregator now expires the group immediately, if the `groupTimeout` is evaluated to a negative value.
+Only `null` is considered as a signal do nothing for the current message.
+
+See <<aggregator>> for more information.
+
+==== @Publisher annotation changes
+
+Starting with _version 5.1, the `@Publisher` AOP functionality has to be turned on explicitly via `@EnablePublisher` or via `<int:enable-publisher>` sub-element on the `<int:annotation-config>`.
+
+See <<publisher-annotation>> for more information.
+
+=== AMQP Changes
 
 `ID` and `Timestamp` header mapping changes in the `DefaultAmqpHeaderMapper`.
 See the note near the bottom of <<amqp-message-headers>> for more information.
@@ -59,15 +72,8 @@ See the note near the bottom of <<amqp-message-headers>> for more information.
 The `contentType` header is no longer incorrectly mapped as an entry in the general headers map.
 See <<amqp-content-type>> for more information.
 
-==== JDBC Changes
+=== JDBC Changes
 
 A confusing `max-rows-per-poll` property on the JDBC Inbound Channel Adapter and JDBC Outbound Gateway has been deprecated in favor newly introduced `max-rows` property.
 
 See <<jdbc>> for more information.
-
-==== Aggregator Changes
-
-An aggregator now expires the group immediately, if the `groupTimeout` is evaluated to a negative value.
-Only `null` is considered as a signal do nothing for the current message.
-
-See <<aggregator>> for more information.