DistroReleaseQueue.status could use the immutable flag

Salgado also suggested:

> This seems a bit evil to me. Have you checked with Andrew if there isn't a
> better way of doing this? Maybe having the DB column as a private attribute
> (not declared in the interface) and then providing a read-only property that
> only reads this private attribute. This way you won't need this _set_status()
> method and all the set_foo() methods could simply do "self._status = foo".

I'll implment what fits better, just after catch up the previous review comments

Not sure if it produces a better behaviour for this class, since selectBy() gets really broken by this change.

The way this sort of problem is fixed now is to use storm_validator on the model's property declaration.

e.g. from IArchive:

{{{
    name = StringCol(
        dbName='name', notNull=True, storm_validator=_validate_archive_name)
}}}

_validate_archive_name() is a method that does this:

{{{
    def _validate_archive_name(self, attr, value):
        if not self._SO_creating:
            assert self.is_copy, "Only COPY archives can be renamed."
        assert valid_name(value), "Invalid name given to unproxied object."
        return value
}}}