[IMP] base/ir/ir_sequence : New subsequences for date ranges and convert to new api

Use `.ids` here. `._ids` is the raw ids list and may contain placeholder pseudo-ids (for records being created).

Also not sure what's the point really, the loop is going to create an entry for each record in self so that's redundant. `dict.fromkeys` is useful when not all iterations will set a value for the corresponding record.