protected String getClientIndex(Client client) {
        return getClientIndex(client.getOrganizationId(), client.getClientId(), client.getVersion());
    }
Generates an in-memory key for an client, used to index the client for later quick
retrieval.
@param client an client
@return a client key
gateway/engine/core/src/main/java/io/apiman/gateway/engine/impl/InMemoryRegistry.java