Don't apply multiple session configurations

Add additional @ConditionalOnMissingBean guards to session
configurations to ensure that the first wins. Also reorder imports
to prefer Redis over JDBC.

See gh-5158

spring-boot-samples/spring-boot-sample-session-redis/src/test/java/sample/session/redis/SampleSessionRedisApplicationTests.java

@@ -56,6 +56,7 @@ public class SampleSessionRedisApplicationTests {
 			if (!redisServerRunning(ex)) {
 				return;
 			}
+			throw ex;
 		}
 
 		URI uri = URI.create("http://localhost:" + port + "/");