public interface Checker

A local interface we can use to make "header checker" objects which can be used by HeaderIterators to filter results. This really SHOULD be done with anonymous classes:

 public void getHeadersByRole(final String role) {
     return new HeaderIterator() {
         public boolean checkHeader(SOAPHeaderBlock header) {
             ...
             if (role.equals(headerRole)) return true;
             return false;
         }
     }
 }

...but there appears to be some kind of weird problem with the JVM not correctly scoping the passed "role" value in a situation like the above. As such, we have to make Checker objects instead (sigh).