Using year to make Tax Rate Amount IDs readable.

Using years in the IDs allows it to be human readable without referring to the definition, it also allows historic coverage to be added at a later date if required.

If the rate changed twice in the same year, the second amount appends the month to the ID e.g. lt_vat_standard_2009_09. lt_vat_standard_2009 cannot be lt_vat_standard_2009_01 as we might not have known about the September rate at the time and once the amount id has been set it can't be changed.