From bd516a491b82c32a46d2f9ed80fffa2360698a38 Mon Sep 17 00:00:00 2001
From: Christian Schwarz <christian@neon.tech>
Date: Wed, 10 May 2023 18:00:52 +0200
Subject: [PATCH] THE PLAN
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- Timeline::get calls reconstruct data ⇒ turn that into a Pin<Box<dyn Future…>> .
- Problem there: we call layer.get_reconstruct_data while holding layer map lock ⇒ it’s a std rwlock ⇒ need to turn it into tokio mutex
- Problem there: we sometimes hold the Tenant::timelines std mutex while holding layer map lock (branching?) ⇒ need to turn it into tokio mutex
- Prolem there: tenant.rs `self.state.send_modify(|| { ... timelines.lock().await })`;
  Can't await inside the closure, and we don't have async closures.
  And we don't control the tokio::sync::watch API.

So, tackle things in reverse here.