Amount Randomization

[TheBlueMatt]

@tnull raise the point that we should think about amount randomization when routing for privacy at #158 (comment).

I think I agree with him that we shouldn't "just do it" unless we have some concrete evidence of its value - one thing we should look at is if invoices "tend to" have 0 msats where we'd want to randomize in the sub-sat range.