PKIX and verification depth.

[Rod Widdowson]

This is probably for Brent for when he comes back (so there is no urgency).

I'm back trying to build the parsers for the PKIX trust engines that we need
for (static) metadata signature validation.

Deep down amongst the objects I need to construct is a 

org.opensaml.security.x509.impl.BasicPKIXValidationInformation

Which takes an "@Nonnull final Integer depth" as a parameter to its
constructor.  I have to fill this in from the "verifyDepth" attribute of an
element of type PKIXValidationInfoType.  This attribute is not required and
so we need a default.

The strong paradigm we are following in IdP configuration is to not put any
defaults into the parser code (which is where this was defaulted to 1 in
V2).  So I'd like to make that @Nonnull into @Nullable and default the value
to 1 in the bean itself.

However that seems like it might be a pretty dangerous change to make for
people who are not configuring via the V2 legacy trust engines.  So rather
than just make the change I want to check that this is OK.

Thanks

Rod