@Override
            public void onFailure(Exception e) {
                dataVersion = null;
            }
Stores a "dataversion" record in the ES store.  There is only a single one of these.  The
return value of the add will include the version number of the entity.  This version
number is what we use to determine whether our cache is stale.
gateway/engine/es/src/main/java/io/apiman/gateway/engine/es/PollCachingEsRegistry.java