ref(server): Build pipeline-like control flow for processing

[jan-auer]

Refactors the envelope and event processing code by splitting the main processing function into mostly independent pipeline steps that all operate on a state. The state is mutated by each function, which can be thought of as passing it from one pipeline step to the next.

From the top-level process function, it becomes immediately clear, which actions are applied with processing and which are skipped. The stages only required for processing are conditionally compiled.

There is no change in Relay's behavior. However, there are a few changes internally:

• The flow for rate limiting is different. The function enforce_quotas is called on every envelope in the pipeline. This prepares the pipeline for rate limiting of items other than events. At the moment, it skips all envelopes without events and still assumes DataCategory::Error for all events.
• Clock drift correction now runs unconditionally and has been removed from the store normalize processor.
• Processing mode errors if no event payload can be extracted from items that should create an event. This includes minidumps with their placeholders.
• Rate limits are still sent to the project for caching. However, these messages have moved from the global error handler into the processing and upstream branches. This means that cached rate limits no longer update the project.