Metadata support: Supported resolver query semantics; Iteration

[Rod Widdowson]

> > See above. Some of those are possibly relevant for discovery purposes,
but
> > that isn't an IdP use case.
> 
> Yeah, I was mainly concerned about whether we were going to screw people
> using the OpenSAML-J metadata providers to implement discovery
> interfaces.  I assume our CDS uses that, but not sure.

As I recall the current OpenSAML implementation doesn't really help the
current CDS and it has to fall back on iteration. 

I concur with Scott that getting it right for the CDS is a distraction right
now - in a great number of ways its needs are orthogonal to the IdP.

With my CDS hat on I'd say that if we ever get back to a CDS we will make it
do the work with an Iterable<EntityDescriptor> and allow it to reject
configurations where the MetadatProvider doesn'tprovide it. After all this
metadata filtering is about the only work that the DS needs to do...