Use KafkaBinderConfigurationProperties directly

Fixes #548

- Remove redundant property copying
- Make `socketBufferSize` configurable (it wasn't provided as a configuration option)

spring-cloud-stream-docs/src/main/asciidoc/spring-cloud-stream-overview.adoc

@@ -1121,7 +1121,10 @@ If set to `false`, the binder will rely on the partition size of the topic being
 If the partition count of the target topic is smaller than the expected value, the binder will fail to start.
 +
 Default: `false`.
-
+spring.cloud.stream.kafka.binder.socketBufferSize::
+  Size (in bytes) of the socket buffer to be used by the Kafka consumers.
++
+Default: `2097152`.
 
 ==== Kafka Consumer Properties