Add mode to disallow unprofitable leases

[benthecarman]

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

Currently you can need to guestimate if your lease will profitable or not dependent on on-chain fees and the execution fee vs the rate you get.

Describe the solution you'd like

It would be nice if there way a flag in the config to only sign leases that net a profit (maybe even a min profit). This should remove some of the guessing game needed and give users more peace of mind that they won't lose money besides on the back half when closing the channel.