compute_tools: disable fast path for safekeeper sync

[erikgrinaker]

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Problem

In #9259, we found that the check_safekeepers_synced fast path could result in a lower basebackup LSN than the flush_lsn reported by Safekeepers in VoteResponse, causing the compute to panic once on startup.

This would happen if the Safekeeper had unflushed WAL records due to a compute disconnect. The TIMELINE_STATUS query would report a flush_lsn below these unflushed records, while VoteResponse would flush the WAL and report the advanced flush_lsn. See https://github.com/neondatabase/neon/issues/9259#issuecomment-2410849032.

Summary of changes

Disable the check_safekeepers_synced fast path. Discussion in https://github.com/neondatabase/neon/issues/9259#issuecomment-2417314252 indicates that it has questionable value and may need further improvements.

There is also a separate fix to flush the Safekeeper WAL on disconnect: https://github.com/neondatabase/neon/pull/9436.

Checklist before requesting a review

• [ ] I have performed a self-review of my code.
• [ ] If it is a core feature, I have added thorough tests.
• [ ] Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• [ ] If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• [ ] Do not forget to reformat commit message to not include the above checklist