Write-tile-if-changed should account for zipfile metadata

[zerebubuth]

The store.write_tile_if_changed function tries to reduce the number of writes to the store by fetching any existing tile at that coordinate from the store and only writing the new tile if they differ. Formats such as JSON or MVT contain only the data about the content of the tile, and metadata such as the file timestamp is transmitted in the HTTP headers, which aren't used in the test of whether the tiles differ.

The zipfile format used by the metatiles stores such metadata internally, meaning that if two tiles are generated at different times then they will differ, even if the tile contents are identical. This prevents the write_tile_if_changed optimisation from working.

Instead, we should add a special case for comparison of two zip files which compares only the set of tiles in the file and their contents.

[zerebubuth]

Fixed in #153.