protected void invalidateCache() {
        synchronized (mutex) {
            clientCache.clear();
            apiCache.clear();
        }
    }
Called to invalidate the cache - clearing it so that subsequent calls to getApi()
or getContract() will trigger a new fetch from the ES store.
gateway/engine/es/src/main/java/io/apiman/gateway/engine/es/CachingEsRegistry.java