Suport date filters

[prashtx]

I think there's some cache mis-utilization here. It looks like we'll still check the full range of entries for count and last addition. For tiles that represent just the last 24 hours of data, that will usually work but will be an inefficient query. If the client requests historical tiles, and if there have been new entries after that time range, then we will misreport that the data has changed. If the client is likely to issue requests for time periods that end "now", then we're also caching tiles that will likely never be requested again, and we're querying the cache for tiles that almost certainly don't exist.

I don't think we'll serve stale tiles, so this is a performance/storage risk and not a bad-data risk. I'm inclined to track it in a separate issue, since it might require non-trivial changes, and since I kind of want to show off the date filter functionality soon.

[hampelm]

Re cache -- I agree that we should file it for a future improvement.