Compression middleware turned out to be hard to use for image cases - it
simply looks at the content-encoding, and if not set, tries to compress
if accepted by the client.
Instead, now individual routes are configured with either that
middleware, or for tiles, I decompress and optionally recompress if
applicable.
Now encoding is tracked separately from the tile content, making it
cleaner too. Plus lots of tests for mbtiles & pmtiles.
Fixes#577
