INT-4566: R2DBC Outbound Channel Adapter

JIRA: https://jira.spring.io/browse/INT-4566

* Fixed review comments
* added `DELETE` and `Criteria` implementation for outbound channel adapter
* Clean up code style
* Add initial docs

src/reference/asciidoc/index-single.adoc

@@ -57,6 +57,8 @@ include::./mongodb.adoc[]
 
 include::./mqtt.adoc[]
 
+include::./r2dbc.adoc[]
+
 include::./redis.adoc[]
 
 include::./resource.adoc[]

src/reference/asciidoc/index.adoc

@@ -38,6 +38,7 @@ This documentation is also available as single searchable link:index-single.html
 <<./mail.adoc#mail,Mail Support>> ::
 <<./mongodb.adoc#mongodb,MongoDb Support>> ::
 <<./mqtt.adoc#mqtt,MQTT Support>> ::
+<<./r2dbc.adoc#r2dbc,R2DBC Support>> ::
 <<./redis.adoc#redis,Redis Support>> ::
 <<./resource.adoc#resource,Resource Support>> ::
 <<./rmi.adoc#rmi,RMI Support>> ::

src/reference/asciidoc/r2dbc.adoc

@@ -0,0 +1,34 @@
+[[r2dbc]]
+== R2DBC Support
+
+Spring Integration provides channel adapters for receiving and sending messages by using reactive access to databases via https://r2dbc.io/[R2DBC] drivers.
+
+You need to include this dependency into your project:
+
+====
+.Maven
+[source, xml, subs="normal"]
+----
+<dependency>
+    <groupId>org.springframework.integration</groupId>
+    <artifactId>spring-integration-r2dbc</artifactId>
+    <version>{project-version}</version>
+</dependency>
+----
+
+.Gradle
+[source, groovy, subs="normal"]
+----
+compile "org.springframework.integration:spring-integration-r2dbc:{project-version}"
+----
+====
+
+[[r2dbc-inbound-channel-adapter]]
+=== Inbound Channel Adapter
+
+TBD
+
+[[r2dbc-outbound-channel-adapter]]
+=== Outbound Channel Adapter
+
+TBD

src/reference/asciidoc/whats-new.adoc

@@ -20,6 +20,11 @@ If you are interested in more details, see the Issue Tracker tickets that were r
 The standalone https://projects.spring.io/spring-integration-kafka/[Spring Integration Kafka] project has been merged as a `spring-integration-kafka` module to this project.
 See <<./kafka.adoc#kafka,Spring for Apache Kafka Support>> for more information.
 
+==== R2DBC Channel Adapters
+
+The Channel Adapters for R2DBC database interaction have been introduced.
+See <<./r2dbc.adoc#r2dbc,R2DBC Support>>  for more information.
+
 [[x5.4-general]]
 === General Changes