pageserver: optimize heatmap downloads by passing period/next_upload in the heatmap

[jcsp]

in #6123, the DOWNLOAD_FRESHEN_INTERVAL constant controls how often we will poll remote storage for new content in a secondary location.

The upload interval for heatmaps is configured externally. We could also configure the download period externally, but it isn't necessary: since the secondary downloader reads the heatmap, we can pass the period information from the uploader to the downloader via that channel.