Properly clears the scopes (#2280)

without this fix when calling `maybeScope(null)` or `withSpan(null)` or `newScope(null)` we ended up with creating a null scope that would allocate additional resources and not clear things.

with this fix we're clearing all the scopes and removing thread locals

spring-cloud-sleuth-api/src/main/java/org/springframework/cloud/sleuth/CurrentTraceContext.java

@@ -94,6 +94,13 @@ public interface CurrentTraceContext {
 	 */
 	interface Scope extends Closeable {
 
+		/**
+		 * Noop instance.
+		 */
+		Scope NOOP = () -> {
+
+		};
+
 		@Override
 		void close();
 

spring-cloud-sleuth-api/src/main/java/org/springframework/cloud/sleuth/Tracer.java

@@ -170,6 +170,13 @@ public interface Tracer extends BaggageManager {
 	 */
 	interface SpanInScope extends Closeable {
 
+		/**
+		 * Noop instance.
+		 */
+		SpanInScope NOOP = () -> {
+
+		};
+
 		@Override
 		void close();