Previously, we'd always add up to the maximum number of hop hints
(and beyond!) when selecting hop hints in our first pass. This
change updates hop hint selection to always stick to our hop hint
limit, and to the "hop hint factor" that we scale our invoices by.
This change will result in selecting fewer channels in our first
pass if their total inbound capacity reaches our hop hint factor.
This prevents us from revealing as many private channels as
before, but has the downside of providing fewer options for
payers.