Fix a rare crash from feature lifetime validation.
This is only reproducible for protos that trigger the lazy build of another proto in the pool that needs feature lifetimes validated. If an error is encountered, we'll end up throwing the lazy build's descriptors into the deferred validation map, but then rolling it back with the original proto. This results in a use-after-free crash.
Except for when it's set to 0 for the immutable empty SmallSortedMap, but the number doesn't change anything for that immutable empty map (it's always empty, it will never hit the max).
This should save 4 bytes of memory per SmallSortedMap.
Future-proof edition 2023 against feature value support windows.
This "feature" hasn't been implemented yet, but this puts a placeholder down to prevent compatibility issues in future editions. Once we provide versioning support on individual feature values, we don't want them becoming usable from edition 2023 protos
