tickets/SP-1623: refactor metadata storage in the prenight scheduler simulation archive to improve performance

[ehneilsen]

This modification creates a cache of metadata in an hdf5 file in the same bucket as the sims.
When querying for metadata, the code now checks the cached metadata, and looks in the cache for metadata for any dates before the last one in the cache. For dates of the last one in the cache or later, it falls back to the older behavior of lookinng in the yaml metadata files for each sim. ("Of the last date" rather than "after the last date" in case the cache does not include all sims from the last date.)
A separate cron job updates the cache daily.
A corresponding branch of schedview_notebooks causes the prenight_sims notebook to use the cache.