storage: consider adding delete_prefix

[tzemanovic]

Akin to prefix_iter, for dealing with e.g. lazy collections it would be convenient to be able to delete anything with a matching storage prefix. We'd need to do this via WriteLog, but when the WriteLog is being committed to DB, there's delete_range in RocksDB that can potentially be used to then apply it.

This would be helpful for finalize-block's apply_inflation rewards accumulator added in #714.