Synced flush was a brilliant idea. It supports instant recoveries with a
quite small implementation. However, with the presence of sequence
numbers and retention leases, it is no longer needed. This change
removes it from 8.0.
Relates #5077
The current snippets in the synced flush docs can cause conflicts with
other background syncs, such as the global checkpoint sync or retention
lease sync, in the docs tests.
This skips tests for those snippets to avoid conflicts.
Nhat Nguyen
James Rodewig
Yannick Welsch