util-varexport: only traverse methods once

[youknowjack]

Copied from an Indeed internal improvement request:

VarExporter.export traverses and exports @Export-annotated methods more than once in some scenarios. Make it stop doing that.

In particular, this code collects a set of all superinterfaces and calls getMethods on each of them (if declaredFieldsOnly is false). That results in O(NumberOfInterfaces) export attempts per method in the worst case:

        Set<Class<?>> classAndInterfaces = Sets.newHashSet();
        getAllInterfaces(c, classAndInterfaces);
        classAndInterfaces.add(c);
        for (Class<?> cls : classAndInterfaces) {
            for (final Method method : (declaredFieldsOnly ? cls.getDeclaredMethods() : cls.getMethods())) {
                Export export = method.getAnnotation(Export.class);
                if (Modifier.isStatic(method.getModifiers())) {
                    loadMemberVariable(method, export, c, true, prefix, null);
                } else {
                    loadMemberVariable(method, export, obj, true, prefix, null);
                }
            }
        }