Allow to cache on_change calls
Following https://discuss.tryton.org/t/cache-management-for-rpc/215
It will be better to call directly on_change* methods instead of using a grouped call if only one field is involved. This would allow to cache the result.
But also we should not have the id in the call otherwise the arguments are always different and there is no benefit for caching.
The id is useful in case of missing value (which should not happen) so to be safe we could simply add 'id' as an argument to all on_change except if the RPC has cache.