Auto-fees enhancement: fee-rate constraints

[dekdynamics]

Is your feature request related to a problem? Please describe.

There are some channels that I never want to charge more than 2500 ppm on, and other channels that I never want to drop to 0 ppm. Sometimes I want to set a global range, and sometimes I have specific channels that need a more, or less restrictive range.

Describe the solution you'd like

1. Global fee-rate constraints

I would like the ability to set upper and lower bounds for auto-fees globally. Perhaps I want the upside ppm to be infinite (don't restrict), but I also want 100ppm as my lowest fee rate. Auto-fees would not adjust fee rates outside of the configured range. Ideally, a global constraints option would be two input boxes: Upper limit, lower limit. Entering a value in the upper limit box (in ppm) would limit all channels to a maximum ppm. Entering a value in the lower limit box (in ppm) would limit all channels to a minimum ppm. Leaving a box blank would mean dont restrict.

2. Per-channel constraints

Imagine I have a channel to my friend, and a channel to LOOP. For my friendly channel, I want to use the global constraints that I have set: 1000 max, 10 min. For that channel, I would not set any per-channel constraints. For my LOOP channel, a 1000 ppm max is way too restrictive, and more importantly, a 10 ppm minimum is way too low. For my LOOP channel, I want to set a per-channel constraint so that the upper limit is infinite, and the lower limit is 2000 ppm.

[levmi]

Thanks for the very detailed issue. Greatly appreciated! We'll take a look at this in coming sprints and keep you updated!