This is what LND does, and it's better for upper layers than trying to
twist our maxfeepercent / exemptfee heuristics to suit.
(I don't remember who complained about this, sorry!)
I'm doing this now because I want to add YA parameter next!
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Added: JSON-RPC: `pay` has new parameter `maxfee` for setting absolute fee (instead of using `maxfeepercent` and/or `exemptfee`)