ref(spool): Make EnvelopeBuffer a Service

[jjbayer]

From the linked ticket:

We decided not to write EnvelopeBuffer as a service because we did not want to put an additional queue in front of it (every service has an inbound queue).

However, since we have multiple request handler threads, we need some kind of synchronization when pushing to the buffer, so in the end we always end up with contention i.e. queueing, whether we queue threads, tokio tasks, or messages. The current implementation spawns a tokio task for every push and synchronizes the buffer with a tokio mutex).

The current solution is in no way better than having a service, so we might as well wrap the envelope buffer in a service and remove the tokio::spawn and mutex.

New data flow:

flowchart TD
    RequestHandler --> |Envelope| BufferService
    BufferService --> |Envelope| ProjectCache 
    ProjectCache --> |"Ready(ProjectKey)"| BufferService
    ProjectCache --> |"NotReady(ProjectKey, Envelope)"| BufferService

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Fixes https://github.com/getsentry/team-ingest/issues/516.

[iambriccardo]

LGTM, I like the new select mechanism. Isn't there a tokio primitive that can function in place of wait_for_changes?