This ensures that we avoid any unexpected conditions inherent in
transferring non-empty mempools across chainstates.
Note that this should never happen in practice given that snapshot
activation will not occur outside of IBD, based upon the height checks
in `loadtxoutset`.
This is a partial backport of core#27596
https://github.com/bitcoin/bitcoin/pull/27596/commits/bb0585779472962f40d9cdd9c6532132850d371c
Depends on D17897