Intent to backport OAK-7157 to 1.8

[Francesco Mari]

I intend to backport OAK-7157 to the 1.8 branch. The fix implements an
optimisation for cold standby instances. With the fix in place,
standby instances only retained the latest generation, instead of the
last two generations. This allows a cold standby instance to remove
old segments more aggressively and save space on disk.