Staking miner: check previous solutions without scraping the entire pallet data.

[kianenigma]

yeah this is a good improvement. Also a critical improvement. is that currently we need to scrape the data of the whole pallet to check if we have a solution in this round or not (since we pass the entire ext), this can be done much better by just querying the appropriate storage item (see ensure_no_previous_solution).

we should be able to early-exit from this check, without the need to scrape the entire ext.

Originally posted by @kianenigma in #3628 (comment)

[kianenigma]

This is already described as a part of #3740 and will be closed with that.