add cache-control of max-age

[miurahr]

need feature to return cache-control: max-age parameter.

In mod_tile:

        /* Test if the tile we are serving is out of date, then set a low maxAge*/
        if (state == tileOld) {
            holdoff = (scfg->cache_duration_dirty / 2) * (rand() / (RAND_MAX
                    + 1.0));
            maxAge = scfg->cache_duration_dirty + holdoff;
        } else {
            // cache heuristic based on zoom level
            if (cmd->z > tile_config->maxzoom) {
                minCache = 0;
                ap_log_rerror(APLOG_MARK, APLOG_INFO, 0, r,
                        "z (%i) is larger than MAXZOOM %i\n", cmd->z, tile_config->maxzoom);
            } else {
                minCache = scfg->mincachetime[cmd->z];
            }
            // Time to the next known complete rerender
            planetTimestamp = apr_time_sec(getPlanetTime(r)
                    + apr_time_from_sec(PLANET_INTERVAL) - r->request_time);
            // Time since the last render of this tile
            lastModified = (int) (((double) apr_time_sec(r->request_time
                    - finfo->mtime))
                    * scfg->cache_duration_last_modified_factor);
            // Add a random jitter of 3 hours to space out cache expiry
            holdoff = (3 * 60 * 60) * (rand() / (RAND_MAX + 1.0));

            maxAge = MAX(minCache, planetTimestamp);
            maxAge = MAX(maxAge, lastModified);
            maxAge += holdoff;

            ap_log_rerror(
                    APLOG_MARK,
                    APLOG_DEBUG,
                    0,
                    r,
                    "caching heuristics: next planet render %ld; zoom level based %ld; last modified %ld\n",
                    planetTimestamp, minCache, lastModified);
        }

        maxAge = MIN(maxAge, scfg->cache_duration_max);
    }

    ap_log_rerror(APLOG_MARK, APLOG_DEBUG, 0, r, "Setting tiles maxAge to %ld\n", maxAge);

    apr_table_mergen(t, "Cache-Control",
                     apr_psprintf(r->pool, "max-age=%" APR_TIME_T_FMT,
                     maxAge));
    timestr = apr_palloc(r->pool, APR_RFC822_DATE_LEN);
    apr_rfc822_date(timestr, (apr_time_from_sec(maxAge) + r->request_time));
    apr_table_setn(t, "Expires", timestr);
}