Alex:
don’t read from db data which in _visubleFiles already. It will allow Prune to be inconsistent.

Example: if keys are Step+TxHash - then prune(step=1) alphabetically - even that txHashes belong to different blocks. (Now we prune block-by-block to keep data consistent).
but i don't know how to achieve it - if you don't have history files (history disabled or pruned)

lexicographic prune can be faster; but will cause inconsistencies in read.

This task gets us closer to be able to do lexicographic prune; but perhaps not all the way