[PP-2] Add async query execution support to views

Why does this soft-depend on 📌 Add PHP Fibers support to BigPipe RTBC ? Because without it, there's no place that the effect would be visible? 🤔

Yeah exactly. If we add support here, and BigPipe has Fibers support, then we should be able to see real life performance improvements.

📌 Add a cache prewarm API and use it to distribute cache rebuids after cache clears / during stampedes Needs work could make use of this too, and once we have async queries in core, it'll potentially work in the single request cold cache case, but that's a bit further from being ready still.

Also postponed on ✨ [PP-1] Async database query + fiber support Active