Miner should abort waiting for proposals

[kantai]

The nakamoto miner should behave a little less strictly when it comes to waiting for proposals:

If the miner sees that they have 20% rejections, and have already waited, e.g., 2 minutes, they should consider their proposal rejected. We could probably come up with some smooth function over the range [0, 30] which defines how long the miner should be waiting. If they haven't received any rejections, then wait forever (as is the current behavior), and if they've received >30% rejections, then abort immediately (as is the current behavior). In between, we want to do something more heuristic.

My guess is that we want something squared:

10 - 10*(x/30)^2

Or just something stepped:

0-10% rejection: wait 10 minutes
10-20% rejection: wait 5 minutes
20-30% rejection: wait 2.5 minutes

[kantai]

Closed by #5731