## Problem
If a caller times out on safekeeper timeline deletion on a large
timeline, and waits a while before retrying, the deletion will not
progress while the retry is waiting. The net effect is very very slow
deletion as it only proceeds in 30 second bursts across 5 minute idle
periods.
Related: https://github.com/neondatabase/neon/issues/10265
## Summary of changes
- Run remote deletion in a background task
- Carry a watch::Receiver on the Timeline for other callers to join the
wait
- Restart deletion if the API is called again and the previous attempt
failed
