@Override
    public List<MapDefinition> getMapDefinitions() {
        return refDataValueProxies.stream()
                .flatMap(refDataValueProxy -> refDataValueProxy.getMapDefinitions().stream())
                .collect(Collectors.toList());
    }
A wrapper for multiple {@link RefDataValueProxy} objects. A lookup (consumeBytes or supplyValue)
will perform a lookup against each sub-proxy in turn until a value is found.
<p>
This is used when we have multiple ref loaders attached to an XSLT. Any one of them may be
able to provide the value. This means the order of ref loaders is important for performance.
The one most likely to yield results should be at the top.
stroom-pipeline/src/main/java/stroom/pipeline/refdata/store/MultiRefDataValueProxy.java