TrustEngine refactoring

[Rod Widdowson]

> I'm not very fluent in the Spring parsing code. I guess what we probably
> should do is actually parse the existing trust engine(s) (which I guess
> we're doing for metadata anyway?) 

We only parse those trust engines which Metadata parsing needs.  If we meet
any of the following we issue log.warn.

MetadataPKIXSignature
MetadataExplicitKeySignature
MetadataPKIXX509Credential
MetadataExplicitKey
Chaining
StaticPKIXX509Credential

I'm guessing that it's some/all of these that need to be parsed. 

> and build:

[Snip]  

It's "just code".  But it needs to be done carefully. 

> But I don't know how much we care. We could hardwire the two chaining
> engines into the system when we parse legacy and just emit warnings like
> with the old security policy stuff.

I don't feel qualified to answer which is the correct thing to do - I'll
defer to want you and Brent decide.
 
> Anyway, we have to do one of those. It won't work right now if we test
> with a legacy RP file.

Rod