private String getApiId(Contract contract) {
        return getApiId(contract.getApiOrgId(), contract.getApiId(), contract.getApiVersion());
    }
Generates a valid document ID for a api referenced by a contract, used to
retrieve the api from ES.
@param contract the contract between client and api
gateway/engine/es/src/main/java/io/apiman/gateway/engine/es/EsRegistry.java