[IMP] account: soft post entries in the future

Add an easy way to not post the entries in the future when calling
post() on it, but rather set it to be auto-posted at accounting date.
This is useful when we are creating a lot of entries in batch and some
might be in the future, some in the past, and we don't want to separate
that in two batch every time. (asset, accrual, transfer,... )