[#255] ir/audit: Make task manager to return number of skipped tasks

Add numeric return from TaskManager.Reset method that shows the number of
canceled tasks. This values will be used for assessment of the progress of
the audit.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/services/audit/taskmanager/reset.go

@@ -1,8 +1,11 @@
 package audittask
 
 // Reset pops all tasks from the queue.
-func (m *Manager) Reset() {
-	for len(m.ch) > 0 {
+// Returns amount of popped elements.
+func (m *Manager) Reset() (popped int) {
+	for ; len(m.ch) > 0; popped++ {
 		<-m.ch
 	}
+
+	return
 }