[Mining] automate RBF and burn fee parameters

[jcnelson]

With the recent burst of mining activity, it seems that miners don't know (or can't be bothered) to calibrate their Bitcoin transaction fees and RBF thresholds. As a result, we have a lot more orphans than we did last week. Namely:

• A low RBF threshold means that miners don't mine off of the latest tip per se. They just mine off of the latest tip they know about, and never try again when the tip gets updated.

• A low transaction fee causes miner block-commits to get bumped to a later block, invalidating them.

I'm opening this issue to discuss the possibility of just requiring miners to set a maximum spend per block, and let the node software figure out what transaction fee and RBF threshold to use based on recent Bitcoin block activity.

[jcnelson]

This is somewhat high-priority given that the inability to do this leads to miner instability.