use the right lock for Enqueue

The request for a non-read lock blocks until all existing read
locks exit, meaning that if an Immediate operation is already
going for a fragment, an Enqueue operation will hang forever
holding the fragment's lock, while the Immediate operation has
probably relinquished the fragment's lock to wait for the
queue worker to process it. But the queue worker can't process
it, because the incoming Enqueue still holds the fragment's
lock. Solution: Don't block the Enqueue operation like that.
It shouldn't coexist with things that actually change the sq
channels, like Stop(), but it is fine for it to coexist with
other queue operations.

snapshotqueue.go

@@ -200,8 +200,8 @@ func (sq *prioritySnapshotQueue) Enqueue(f *fragment) {
 	if f.snapshotPending {
 		return
 	}
-	sq.mu.Lock()
-	defer sq.mu.Unlock()
+	sq.mu.RLock()
+	defer sq.mu.RUnlock()
 	if sq.normal == nil {
 		sq.logger.Printf("requested snapshot after snapshot queue was closed")
 		return