Can't promote a write operation to a mutator operation

[Leonard Richardson]

I'm just filing this bug for bookkeeping purposes. Here's a description from the merge proposal that fixed it:

Let me explain this in terms of the class I used to test it.

    >>> class IOperationPromotedToMutator(Interface):
    ... export_as_webservice_entry()
    ...
    ... field = exported(TextLine(readonly=True))
    ...
    ... @mutator_for(field)
    ... @operation_for_version('3.0')
    ... @operation_parameters(text=TextLine())
    ... @export_write_operation()
    ... @operation_for_version('1.0')
    ... def set_value(text):
    ... pass

The example web service has four versions: beta, 1.0, 2.0, and 3.0. In 'beta', mutator methods were also published as named operations (just like in Launchpad 'beta'), but that behavior stopped in 1.0. Call 1.0 the 'cutoff version'.

The 'set_value' method is introduced in 1.0, and it's a normal write operation in 2.0, but in 3.0 it's promoted to a mutator method.

Here's the problem: this method was not available in 1.0 or 2.0. lazr.restful saw that set_value was defined as a mutator, and added a mask registration to stop it from showing up as of the 'cutoff version', 1.0. This would have been the correct behavior if set_value was defined as a mutator *before* the cutoff version, but actually it was defined as a mutator *after* the cutoff version.

lazr.restful was masking the write operation registration in the cutoff version if the method was defined as a mutator in *any* version. This branch changes it so that the operation is only masked in the cutoff version if it was defined as a mutator *before* the cutoff version. If a method is promoted to a mutator after the cutoff version, a lookup for that method will fail starting in the promotion version.

This branch also changes the versioned request marker interfaces in webservice-declarations.txt so that version n+1 is a subclass of version n. This more closely reproduces the behavior of a real lazr.restful web service, and lets me do a more realistic test.