Cache eviction and pinning

[kangas]

The initial cache implementation (#7) will likely just be a persistent key-value store with no eviction. Eventually, we'll have to evict cache entries.

Distinguish between:

• TTL: how long the content should be considered "fresh"
  • Example: daily full index should be refetched after 24 hrs, and its derived artifacts (secondary indexes) regenerated
• Date of recent use: for LRU eviction
• Saved/pinned
  • Quarterly "full" indexes should not be evicted
  • User "starred" documents should not be evicted

Related reading:

https://r.va.gg/2013/10/should-i-use-a-single-leveldb-or-many-to-hold-my-data.html
https://r.va.gg/presentations/lxjs2013/