rebalance: allow max fee to be set in absolute terms (msat or sat)

[billygarrison]

Currently the fee ratio for rebalance feels unintuitive, I recommend allowing an optional parameter, perhaps maxfeeabsolute where you can specify "I am only willing to pay 5sat to balance this channel"

rebalanceall is a bit harder to implement maxfeeabsolute, but you could still use it and then if any of the rebalances would go above that amount then it doesn't do the rebalance.

[billygarrison]

A related solution for rebalance for individual channels could be to prompt the user: "Route found, rebalancing will cost 3500msat. Continue?"

[manreo]

I think it actually already exists.. Although not very intuitive, the parameter exemptfee will do that, if one sets maxfeepercent to 0, then exemptfee acts as your suggested maxfeeabsolute