Rewrite client HTTP cache again

[danielballan]

• [ ] Refactor to use hischel, which unforunately emerged just after the last rewrite of tiled.client.cache. It takes the same approach as that rewrite does, interposing itself at the HTTP transport level. It has more a mature, standards-compliant implementation respecting HTTP cache control headers.
• [ ] Hischel supports pluggable storage, including built-in support for Redis (of interest to @dylanmcreynolds) and SQLite. In previous discussions with @nmaytan, I believe we considered implementing a custom storage plugin for Hischel that would let us keep our existing on-disk format, which is SQLite-based but has a different table structure. As I recall, our reasoning was that we wanted to isolate ourselves from Hischel to an extent, with direct control over our persisted artifacts. Taking a fresh look at that now, given that the artifact is transient anyway, I might be inclined to accept Hischel's built-in SQLite format.
• [ ] Review the way that the Tiled server handled cache-control headers. Evaluate actual performance savings with and without cache.