accounting: isolate stats to groups

Introduce stats groups that will isolate accounting for logically
different transferring operations. That way multiple accounting
operations can be done in parallel without interfering with each other
stats.

Using groups is optional. There is dedicated global stats that will be
used by default if no group is specified. This is operating mode for CLI
usage which is just fire and forget operation.

For running rclone as rc http server each request will create it's own
group. Also there is an option to specify your own group.

fs/operations/multithread_test.go

@@ -60,8 +60,8 @@ func TestMultithreadCopy(t *testing.T) {
 
 			src, err := r.Fremote.NewObject(context.Background(), "file1")
 			require.NoError(t, err)
-			accounting.Stats.ResetCounters()
-			tr := accounting.Stats.NewTransfer(src)
+			accounting.GlobalStats().ResetCounters()
+			tr := accounting.GlobalStats().NewTransfer(src)
 
 			defer func() {
 				tr.Done(err)