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-redis-json/build.gradle

@@ -28,6 +28,7 @@ dependencies {
             "org.springframework.boot:spring-boot-starter-thymeleaf",
             "org.springframework.boot:spring-boot-starter-security",
             "nz.net.ultraq.thymeleaf:thymeleaf-layout-dialect",
+            "biz.paluch.redis:lettuce:$lettuceVersion",
             "org.apache.httpcomponents:httpclient"
 
     testCompile "org.springframework.boot:spring-boot-starter-test",