ref(spooler): Remove use of legacy project cache

[iambriccardo]

This PR removes the old legacy project cache and implements the forwarding logic directly in the spooler. The new logic works as follows:

• We still optimistically assume that a new project key pair is ready. (This assumption is intentionally made without the sync check to simplify the buffering code)
• When peeking, if the key pair is ready, we synchronously load the project states. If at least one state is pending, the pop will not occur, and the project configs will be scheduled for fetching. If both states are available, the envelope will either be forwarded to the processor or dropped, depending on the state.

[iambriccardo]

I decided that I would prefer to encode in the pair the lack of sampling key and rather expose different methods for making it behave like previously, where they were both set and the fallback of sampling_key was own_key.

[iambriccardo]

I decided that I would prefer to encode in the pair the lack of sampling key and rather expose different methods for making it behave like previously, where they were both set and the fallback of sampling_key was own_key.