Consider adding optimistic memoize-cache refresh for latest tokenizer batch versions

[dlongley]

Whenever the cached batch version expires for a tokenizer, a hit to the database occurs during the lookup. Ideally, the cache would use memoized promises instead and, shortly before the record expires, a background call would be made to look up the batch version and refresh it in the cache (provided the cache had room, etc.).

[dlongley]

Ideally, this feature would be added to a library like lru-memoize so it could be reused here (and elsewhere as needed).