## Problem
- `test_basebackup_cache` fails in
https://github.com/neondatabase/neon/pull/11712 because after the
timelines on safekeepers are managed by storage controller, they do
contain proper start_lsn and the compute_ctl tool sends the first
basebackup request with this LSN.
- `Failed to prepare basebackup` log messages during timeline
initialization, because the timeline is not yet in the global timeline
map.
- Relates to https://github.com/neondatabase/cloud/issues/29353
## Summary of changes
- Account for `timeline_onto_safekeepers` storcon's option in the test.
- Do not trigger basebackup prepare during the timeline initialization.
## Problem
Basebackup cache is on the hot path of compute startup and is generated
on every request (may be slow).
- Issue: https://github.com/neondatabase/cloud/issues/29353
## Summary of changes
- Add `BasebackupCache` which stores basebackups on local disk.
- Basebackup prepare requests are triggered by
`XLOG_CHECKPOINT_SHUTDOWN` records in the log.
- Limit the size of the cache by number of entries.
- Add `basebackup_cache_enabled` feature flag to TenantConfig.
- Write tests for the cache
## Not implemented yet
- Limit the size of the cache by total size in bytes
---------
Co-authored-by: Aleksandr Sarantsev <aleksandr@neon.tech>
