Ensures Span.timestamp/duration are not reported on remote spans (#443)

This ensures spans that have remote set to true don't send
Span.timestamp/duration.

In the RPC span model, the client owns the timestamp and duration of the
span. If we were propagated an id, we can assume that we shouldn't
report timestamp or duration, rather let the client do that. Worst case
we were propagated an unreported ID and Zipkin backfills timestamp and
duration.

spring-cloud-sleuth-samples/spring-cloud-sleuth-sample-messaging/src/test/java/integration/MessagingApplicationTests.java

@@ -114,7 +114,7 @@ public class MessagingApplicationTests extends AbstractIntegrationTest {
 		Optional<Span> eventReceivedSpan = findSpanWithAnnotation(Constants.CLIENT_RECV);
 		Optional<Span> lastHttpSpansParent = findLastHttpSpansParent();
 		// "http:/parent/" -> "home" -> "message:messages" -> "http:/foo" (CS + CR) -> "http:/foo" (SS) -> "foo"
-		Collections.sort(this.integrationTestSpanCollector.hashedSpans, (s1, s2) -> s1.timestamp.compareTo(s2.timestamp));
+		Collections.sort(this.integrationTestSpanCollector.hashedSpans);
 		thenAllSpansArePresent(firstHttpSpan, eventSpans, lastHttpSpansParent, eventSentSpan, eventReceivedSpan);
 		then(this.integrationTestSpanCollector.hashedSpans).as("There were 6 spans").hasSize(6);
 		log.info("Checking the parent child structure");