Activation Criteria for Attribute resolvers

[Rod Widdowson]

> How are the AttributeResolutionContext and AttributeRecipientContext
> linked ? By parent context ?

For now the recipient is a child of the AttributeResolutionContext (and
AFAIR the filters get it via this, which is found via their parent).  But
this is very much just to give us something to work against.

I'd imagine that as we start to put the Attribute flows together this will
become much clearer, and some minor rework will be needed.

Rod