pageserver: reorder upload queue when possible (#10218)

## Problem

The upload queue currently sees significant head-of-line blocking. For
example, index uploads act as upload barriers, and for every layer flush
we schedule a layer and index upload, which effectively serializes layer
uploads.

Resolves #10096.

## Summary of changes

Allow upload queue operations to bypass the queue if they don't conflict
with preceding operations, increasing parallelism.

NB: the upload queue currently schedules an explicit barrier after every
layer flush as well (see #8550). This must be removed to enable
parallelism. This will require a better mechanism for compaction
backpressure, see e.g. #8390 or #5415.

Cargo.lock

@@ -4044,6 +4044,7 @@ dependencies = [
  "postgres_connection",
  "postgres_ffi",
  "postgres_initdb",
+ "pprof",
  "pq_proto",
  "procfs",
  "rand 0.8.5",