[#12] mclock: Fix deadlock caused by mclock.Close

Deadlock scenario:
- mclock closed by `Close` method, it locks mutex and calls `clock.close`
- clock starts `scheduleRequest` goroutine, it tries to lock mutex
- `clock.Close` waits for all goroutines

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

scheduling/mclock.go

@@ -137,15 +137,15 @@ func (q *MClock) RequestArrival(ctx context.Context, tag string) (ReleaseFunc, e
 // No new requests for scheduling will be accepted after the closing.
 func (q *MClock) Close() {
 	q.mtx.Lock()
-	defer q.mtx.Unlock()
-
 	q.closed = true
-	q.clock.close()
 	for q.limitQueue.Len() > 0 {
 		item := heap.Pop(q.limitQueue).(*limitMQueueItem)
 		close(item.r.canceled)
 		q.removeFromQueues(item.r)
 	}
+	q.mtx.Unlock()
+
+	q.clock.close()
 }
 
 func validateParams(runLimit uint64, tagInfo map[string]TagInfo) error {
@@ -261,6 +261,10 @@ func (q *MClock) scheduleRequest() {
 	q.mtx.Lock()
 	defer q.mtx.Unlock()
 
+	if q.closed {
+		return
+	}
+
 	q.scheduleRequestUnsafe()
 }