safekeeper: speed up timeline deletion

[erikgrinaker]

We saw a Safekeeper timeline deletion with 12 TB WAL take 30 minutes to complete. We should try to speed this up by about an order of magnitude. Some ideas:

• Use an iterator to list objects instead of re-listing for every batch.
• Send many (e.g. 16) concurrent batch deletion requests.
• Respect the remote storage's max_keys_per_delete (Azure only allows 256, not 1000 like S3).

[jcsp]

Agree this could be made to run faster, but on the other hand deletions can be thought of as a very low priority operation, so it's not necessarily helpful to take more than one unit of remote storage concurrency for them 🤔

[arpad-m]

Earlier issue: #9152