add default span layer

apm-network/src/main/proto/TraceSegmentService.proto

@@ -72,10 +72,11 @@ enum SpanType {
 }
 
 enum SpanLayer {
-    Database = 0;
-    RPCFramework = 1;
-    Http = 2;
-    MQ = 3;
+    Unknown = 0;
+    Database = 1;
+    RPCFramework = 2;
+    Http = 3;
+    MQ = 4;
 }
 
 message LogMessage {

apm-sniffer/apm-agent-core/src/main/java/org/skywalking/apm/agent/core/context/trace/SpanLayer.java

@@ -4,10 +4,10 @@ package org.skywalking.apm.agent.core.context.trace;
  * @author wusheng
  */
 public enum SpanLayer {
-    DB(0),
-    RPC_FRAMEWORK(1),
-    HTTP(2),
-    MQ(3);
+    DB(1),
+    RPC_FRAMEWORK(2),
+    HTTP(3),
+    MQ(4);
 
     private int code;