Add support for negative caching to cache.Cache

[nickstenning]

This commit adds support for negative caching, that is: caching the non-existence of a specific key. This can help to reduce the cost of cache misses when a client is repeatedly requesting a non-existent object, and may also help mitigate denial of service attacks which target the relatively expensive cache fill operation.

The change is backwards compatible with the existing interface for NewCache through the magic of functional options. Negative caching is disabled by default, but can be enabled by specifying a WithNegativeCaching option at initialization:

fresh := 30 * time.Second
stale := 10 * time.Minute
negative := 10 * time.Second

c := cache.NewCache[User](redis, "user", fresh, stale, cache.WithNegativeCaching(negative))

To use negative caching, the cache fetcher function must return a sentinel error value, cache.ErrDoesNotExist. This is also the error that will be returned for a negative result. Clients should use errors.Is(err, cache.ErrDoesNotExist) to identify a negative result.

[philandstuff]

This looks fine. I agree that functional options are the bee's knees (they're sort of CRDTs if you squint).

I think the one thing I feel uneasy about is that non-existence isn't an error and I'd rather not treat it as such. We already enforce that nil cannot be set in the cache, so we could safely return nil to represent nonexistence, which feels more Go-ish.

If you disagree, feel free to merge (after removing the debug print).

[nickstenning]

we could safely return nil to represent nonexistence, which feels more Go-ish.

We discussed this in person. This isn't that easy because T isn't necessarily a pointer. I'll see later if we can make a change that rejects all zero values rather than just literal nils.