Revert "Revert "Add new trace segment for improving sampling performance.""

This reverts commit 5b853ec38b.

apm-commons/apm-trace/src/main/java/org/skywalking/apm/trace/AbstractTraceSegment.java

@@ -0,0 +1,7 @@
+package org.skywalking.apm.trace;
+
+/**
+ * @author wusheng
+ */
+public interface AbstractTraceSegment {
+}

apm-commons/apm-trace/src/main/java/org/skywalking/apm/trace/TraceSegment.java

@@ -19,7 +19,7 @@ import java.util.List;
  * <p>
  * Created by wusheng on 2017/2/17.
  */
-public class TraceSegment {
+public class TraceSegment implements AbstractTraceSegment{
     private static final String ID_TYPE = "Segment";
 
     /**

apm-commons/apm-trace/src/main/java/org/skywalking/apm/trace/UnsampleTraceSegment.java

@@ -0,0 +1,7 @@
+package org.skywalking.apm.trace;
+
+/**
+ * @author wusheng
+ */
+public class UnsampleTraceSegment {
+}