Consider cache auto-updater

[dlongley]

Since fetching some DID docs can be a slow process, we may want to engineer something to keep caches warm independently of resolution requests. What this means is that -- we could schedule a timer to check for cache entries that are close to expiring, and we could re-resolve them in a background process. This would help reduce wait times when popular values have expired from the cache and just need to be refreshed.