fix golint issues in core/threading (#524)

core/threading/workergroup.go

@@ -1,10 +1,12 @@
 package threading
 
+// A WorkerGroup is used to run given number of workers to process jobs.
 type WorkerGroup struct {
 	job     func()
 	workers int
 }
 
+// NewWorkerGroup returns a WorkerGroup with given job and workers.
 func NewWorkerGroup(job func(), workers int) WorkerGroup {
 	return WorkerGroup{
 		job:     job,
@@ -12,6 +14,7 @@ func NewWorkerGroup(job func(), workers int) WorkerGroup {
 	}
 }
 
+// Start starts a WorkerGroup.
 func (wg WorkerGroup) Start() {
 	group := NewRoutineGroup()
 	for i := 0; i < wg.workers; i++ {