Potential race condition for locking flat storage updates

[Longarithm]

There is a FlatStorageInner::move_head_enabled field to lock flat storage updates if we need to make snapshot at the epoch boundary. However, it is set to false only when state snapshot request is sent. It means that if request is extremely late, flat storage will remain unlocked and will pass the epoch boundary, and after that it will be impossible to make epoch boundary snapshot.

We probably don't hit it because the gap between receiving request and passing the epoch is big enough, but the proper solution is to lock and unlock flat storage at the Client side, as it is responsible for flat storage updates.