pkg: create package traceutil for tracing. mvcc: add tracing

steps:range from the in-memory index tree; range from boltdb.
etcdserver: add tracing steps: agreement among raft nodes before
linerized reading; authentication; filter and sort kv pairs; assemble
the response.

pkg/traceutil/trace_test.go

@@ -0,0 +1,28 @@
+package traceutil
+
+import (
+	"testing"
+)
+
+func TestTrace(t *testing.T) {
+	var (
+		op    = "Test"
+		steps = []string{"Step1, Step2"}
+	)
+
+	trace := New(op)
+	if trace.operation != op {
+		t.Errorf("Expected %v, got %v\n", op, trace.operation)
+	}
+
+	for _, v := range steps {
+		trace.Step(v)
+		trace.Step(v)
+	}
+
+	for i, v := range steps {
+		if v != trace.steps[i].msg {
+			t.Errorf("Expected %v, got %v\n.", v, trace.steps[i].msg)
+		}
+	}
+}