Allow to set maximum total fees during routing

[tnull]

We should give users the capability of setting a hard cap for the routing fees, e.g, through PaymentParameters::max_total_routing_fee or similar.

This might also go hand-in-hand with #495, as such an upper bound might give us some indication of a privacy budget we're allowed to spend on finding more reliable or more random routes.

[TheBlueMatt]

We definitely should, but applying this is a bit complicated, at least on the outbound_payment end - we'll need to figure out how much of our budget is left every time we go to retry, and will run the risk of "over-spending" on one path and then running out entirely on another retry. At least on the routing end its probably not that hard to apply, though there's no way to set a total limit on a secondary metric in dijkstras without spuriously failing, but hopefully its low enough risk it doesn't matter.

[tnull]

As stated in #2409:

Also we should rip out the 10% check in the retry code which was really just a buggy code check, but that should happen later.

[TheBlueMatt]

Turned out to be a bit bigger of a refactor than we could fit inside 116, but definitely need to get it in in 117.