## Problem
With upload queue reordering in #10218, we can easily get into a
situation where multiple index uploads are queued back to back, which
can't be parallelized. This will happen e.g. when multiple layer flushes
enqueue layer/index/layer/index/... and the layers skip the queue and
are uploaded in parallel.
These index uploads will incur serial S3 roundtrip latencies, and may
block later operations.
Touches #10096.
## Summary of changes
When multiple back-to-back index uploads are ready to upload, only
upload the most recent index and drop the rest.
## Problem
The upload queue can currently schedule an arbitrary number of tasks.
This can both spawn an unbounded number of Tokio tasks, and also
significantly slow down upload queue scheduling as it's quadratic in
number of operations.
Touches #10096.
## Summary of changes
Limit the number of inprogress tasks to the remote storage upload
concurrency. While this concurrency limit is shared across all tenants,
there's certainly no point in scheduling more than this -- we could even
consider setting the limit lower, but don't for now to avoid
artificially constraining tenants.
## 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.
