Dynamic partition improvements in Kafka binder

Resolves https://github.com/spring-cloud/spring-cloud-stream/issues/2706

  - Update partition count changes in Kafka dynamically during runtime
  - Checkstyle fixes
  - PartitionHandler changes
  - Improved handling of expression with 'payload' in the logic
  - Removed getter/setter for PartitionHandler and use ReflectionUtils within Test
  - Renamed property to 'dynamicPartitionUpdatesEnabled' and improved documentation (also one line per sentence)
  - Improved/renamed test to use embeddedkafka and really test update behaviour
  - Javadoc

core/spring-cloud-stream/src/main/java/org/springframework/cloud/stream/binder/ProducerProperties.java

@@ -81,6 +81,8 @@ public class ProducerProperties {
 
 	private PollerProperties poller;
 
+	private boolean dynamicPartitionUpdatesEnabled = false;
+
 	public String getBindingName() {
 		return bindingName;
 	}
@@ -195,6 +197,24 @@ public class ProducerProperties {
 		this.poller = poller;
 	}
 
+	/**
+	 * Returns status of property dynamicPartitionUpdatesEnabled.
+	 * @return true if dynamic updates should are enabled otherwise false
+	 */
+	public boolean isDynamicPartitionUpdatesEnabled() {
+		return dynamicPartitionUpdatesEnabled;
+	}
+
+	/**
+	 * A flag which enables/disables partition count updates during runtime. Disabled by default.
+	 * Depends on binder if supported or not.
+	 * Currently only supported by kafka binder (see 'Partitioning with the Kafka Binder' documentation for details)
+	 * @param enabled true if dynamic updates should be enabled otherwise false
+	 */
+	public void setDynamicPartitionUpdatesEnabled(boolean enabled) {
+		this.dynamicPartitionUpdatesEnabled = enabled;
+	}
+
 	static class ExpressionSerializer extends JsonSerializer<Expression> {
 
 		@Override