Add random CLTV offsets (or max CLTV offsets) during route calculation

[TheBlueMatt]

As suggested by BOLT 7 in "Recommendations for Routing" to avoid making it obvious which hops a route is taking.

[TheBlueMatt]

Note we should do the same for the value sent.

[TheBlueMatt]

Done in #1286.

[tnull]

Note we should do the same for the value sent.

Do we still want some form of fee randomization/overpayment? On the one hand, adding the exact fees might give something away to an adversary that is able to guess the actual payment amount to be delivered correctly. On the other, it is questionable how often this would be the case and whether these few instances would actually justify the losing some amount in overpaid fees and making the routing logic more complex.

[TheBlueMatt]

Oh good point, yea, maybe lets open an issue to track it, but I think I agree with you there may not be a ton of value in it. One question is if we should randomize in the sub-sat range - we should maybe investigate how much invoices in the wild have 0 msats or any other "round numbers" where we should randomize a bit to add noise.

[TheBlueMatt]

#1482