private String getCacheKey(RoutingContext ctx) {
        StringBuilder ret = new StringBuilder();
        for (int i = 0; i < ctx.getNumRecipients(); ++i) {
            ret.append(ctx.getRecipient(i).getHop(0).toString()).append(" ");
        }
        return ret.toString();
    }
Returns a cache key for this instance of the policy. Because behaviour is based on the recipient list of this
policy, the cache key is the concatenated string of recipient routes.

@param ctx The routing context.
@return The cache key.
documentapi/src/main/java/com/yahoo/documentapi/messagebus/protocol/RoundRobinPolicy.java