DAL/Node: clean-up only for levels with data

What

With this MR, when a non-"refutation supporting" DAL node re-starts, it cleans up the store starting with last_processed_level and up to last_processed_level + storage_period.

It is an alternative to !16864 (closed). The advantages are that (1) it is maybe less error-prone in that it does not need to rely on a new addition to the KVS library (the reset function), and (2) it is a bit more general, as it works also when delta_levels) is not big enough. The disadvantage is that it is slower, as it removes data level by level.

Closes #7771 (closed).

Why

Because for all subsequent levels there is no data, so there is no need to inspects such levels.