[#106] Converted UUID to Long

- Changed Random instantiation to a shared Random
- Changed the name of the converter
- Changed generator into random
- Span id is now non-nullable.
    - it gets generated in the http filter if it's not there
    - it's generated in the spring-integration channels if it wasn't set

spring-cloud-sleuth-samples/spring-cloud-sleuth-sample-messaging/src/main/java/sample/SampleBackground.java

@@ -32,11 +32,12 @@ public class SampleBackground {
 
 	@Autowired
 	private TraceManager traceManager;
+	@Autowired
+	private Random random;
 
 	@SneakyThrows
 	@Async
 	public void background() {
-		final Random random = new Random();
 		int millis = random.nextInt(1000);
 		Thread.sleep(millis);
 		this.traceManager.addAnnotation("background-sleep-millis", String.valueOf(millis));