Use coins and difficulty to estimate time to earn

This change makes the "time to earn reward" estimate much more realistic.

Previously the code was assuming 1-minute blocks, which we just aren't seeing.

Old logic:

"you have weight 1000, network has weight 4000, so you're 25% of the network, so you get 4 minute blocks"...

That's only true if the network gets 1 minute blocks, which it currently doesn't.