Update Samples and Guides to use Lettuce

Favor lettuce because of multiplexing and improved scalability.
Using lettuce requires a fixed number of connections hence using
lettuce improves application scalability.

Fixes gh-652

samples/httpsession-xml/src/main/webapp/WEB-INF/spring/session.xml

@@ -13,6 +13,6 @@
 	<bean class="org.springframework.session.data.redis.config.annotation.web.http.RedisHttpSessionConfiguration"/>
 
 	<!--2-->
-	<bean class="org.springframework.data.redis.connection.jedis.JedisConnectionFactory"/>
+	<bean class="org.springframework.data.redis.connection.lettuce.LettuceConnectionFactory"/>
 	<!-- end::beans[] -->
 </beans>