operations: fix goroutine leak in case of copy retry

Whenever transfer.Account() is called, a new goroutine acc.averageLoop()
is started. This goroutine exits only when the channel acc.exit is closed.
acc.exit is closed when acc.Done() is called, which happens during tr.Done().

However, if tr.Reset is called during a copy low level retry, it replaces
the tr.acc, without calling acc.Done(), which results in the goroutine
mentioned above never exiting.

This commit calls acc.Done() during a tr.Reset()

fs/accounting/transfer.go

@@ -132,6 +132,7 @@ func (tr *Transfer) Reset(ctx context.Context) {
 	ci := fs.GetConfig(ctx)
 
 	if acc != nil {
+		acc.Done()
 		if err := acc.Close(); err != nil {
 			fs.LogLevelPrintf(ci.StatsLogLevel, nil, "can't close account: %+v\n", err)
 		}