Clarify confirmation time stats

[zhyatt]

The confirmation time stats (either from confirmation_history or average from websocket confirmations) can be misleading since they don't sample the time between the block being sent and the time of confirmation.

• At best, it's the delta between receiving the block and its confirmation (although with some added time to commit the cement)
• We use cached votes so some nodes can regularly have confirmation times of 0.
• Elections can drop, and when restarted the confirmation timer starts from zero

Ultimately the only way to accurately know confirmation times is to send a block yourself and listen to its confirmation. Similar to https://repnode.org/network/propagation-confirmation.