Add cache configuration

[nyurik]

Each tile source cache should be configurable, at least to be able to selectively enable/disable cache, but possibly more advanced.

Steps:

• Add a boolean or a tri-state enum to each tile source config to allow cache=enable|disable|auto. The auto setting (default) should be the same as enable for now, but may become more complex in the future.
• Make cache configurable per source type (postgres vs mbtiles vs pmtiles)
• For postgres, make cache configurable for all auto-discoverable tables / functions

[saosebastiao]

I would like to add that a common use case for caching with tile servers is to have Z-level specific caching. For example, large datasets at low zoom levels are very expensive, but at high zoom levels they are really cheap, so it would be nice to be able to say something like : "for data_source_a I want to cache zoom levels from 0-5 only, but for data_source_b I want to cache zoom from 0-9"

[saosebastiao]

Additionally, cache invalidation rules...we should be able to specify a TTL at the very least.

An ideal-world scenario, but obviously far more complex is cache invalidation that happens in sync with the database via LISTEN/NOTIFY updates via trigger functions attached to tables.

[bernatfortet]

I would like to add that a common use case for caching with tile servers is to have Z-level specific caching. For example, large datasets at low zoom levels are very expensive, but at high zoom levels they are really cheap, so it would be nice to be able to say something like : "for data_source_a I want to cache zoom levels from 0-5 only, but for data_source_b I want to cache zoom from 0-9"

This would be amazing to have. In my case, users need to update specific geometries so when that happens the cache needs to be busted. Right now I need to choose between all or northing.