can far category

[Speedy37]

The current spec about far category raise a problem to implement some classes.

For example, the datasource interface save method always does some work locally before calling the real far method distantSave. Both calls uses the same syntax but there is no difference in how they are declared and the parameters behaviors isn't the same.

So a new kind of category must be introduced to still mark "can call a far method" methods: canFarCategory.

The far* invocation methods are renamed inv* and can call far, canFar methods