There is no such thing as a cancelled response, only cancelled requests. A request that is cancelled does not have its callback called with a Response.
This allows the FileSource interface itself to support revalidation. We could (and probably should) now rewrite HTTPContextBase implementations as FileSource implementations.
Added aliases for std::chrono typedefs (eg. 'Seconds' for
std::chrono::seconds). These aliases are used together with templated
helper functions to replace time_t with std::chrono::seconds for most
cases, in particular for 'modified' and 'expires' values in Response.
We're now reparsing tiles when they expire. We're also swapping out buckets atomically to avoid flickering data; i.e. we're displaying the old data as long as we don't have a new parsed bucket for that layer yet. The parsed buckets now live in the *TileData objects rather than in the TileWorker; only partially parsed == pending buckets will remain in the TileWorker. Once they're parsed, they're moved to the *TileData object.
We're now returning stale responses from cache. Those responses will have the `stale` flag set to true. Currently, all requesters in the core code discard stale responses, and cancel the request immediately after they got a non-stale response.
