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.

Follow-up work could include investigating how to reintroduce a bounded buffer between the buffer and the processor to prevent unspooling a large number of envelopes at once, which would accumulate in the processor queue. This option is not strictly necessary due to our memory threshold, which halts unspooling, but it may still be worth exploring.

Closes: #4335

[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 before, where they were both set and the fallback of sampling_key was own_key.