Basic confirmation request loop enhancements

[guilhermelawless]

This is split 1/? (a safe one) from #2306

• Rename variables with suffix _l for local variables in request_confirm as it's a large function
• Rename some variables so their purpose is obvious
• Add callbacks to allow throttling broadcasting. It could happen that more broadcasts would be queued while the previous ones hadn't finished, especially under load
• Add condition node.ledger.block_count_cache > node.ledger.cemented_count + roots.size () + pending_confirmation_height_size before checking frontiers as that can read the disk for 2 seconds when not necessary. It's still not perfect, per the in-file comment, as e.g., 10 roots can be added and they're transformed into a single frontier by the confirmation height processor. Open to ideas on making this condition more consistent.