Paginate redis scan pages properly and don't use Notify anymore for entity caching

[bnjjj]

Calling https://docs.rs/fred/latest/fred/types/trait.Scanner.html#tymethod.next everytime.

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[garypen]

Why 128?

I think this now acts as an informal concurrency limit on the number of in-flight invalidation requests. That's probably desirable, but we should think carefully about the exact number we want and maybe also consider documenting it.

We should make it a global const and use the const in the tests and give it a really good name that explains what it's doing.

[bnjjj]

I just wanted to keep the same configuration than in Notify https://github.com/apollographql/router/blob/bnjjj/fix_redis_scan/apollo-router/src/notification.rs#L33:L33 the previous impl we used. But I think it should be 1024 that's the other const I have to take. I'll create a new const for this