[grouper-shib] improve member data connector cache performance

In version 2.1.0, the member data connector found members via
subject = SubjectFinder.findByIdOrIdentifier()
and
member = MemberFinder.findBySubject(subject)

However, when retrieving subject attributes from the member via
subject.getAttributeValues()
the LazySubject implementation uses
SubjectFinder.findByIdAndSource()
which is a different cache than findByIdOrIdentifier().

As of version 2.1.1, the member data connector finds members via
SubjectFinder.findByIdAndSource()
to re-use the same cache and reduce unnecessary ldap searches.