refactor(s2n-quic-dc): put map impl behind trait

[camshaft]

Description of changes:

This change puts the path secret map behind a trait. This is needed to keep the type concrete while still including a event::Subscriber. My goal was to keep any dyns we have as high up in the tree as possible to maximize the information the compiler has. With this, it can remove functions that aren't subscribed to, or at least inline them to be inexpensive.

Call-outs:

I've spent some time splitting up the map.rs module into various submodules. It was getting quite large and no interfaces were really being enforced so there was quite a bit of refactoring involved.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[Mark-Simulacrum]

This is needed to keep the type concrete while still including a Subscriber

Say more about this? It looks like in #2362 we're making the type Store<S> anyway? Is the plan to pass around Arc<dyn Store> or something like that?

It's not obvious to me that is the better option vs. Arc<dyn Subscriber> (not sure if we have any type aliases on it, but seems like we could build an erased subscriber if needed).