Avoid needless MPP on multiple channels to the same first-hop

Avoid needless MPP on multiple channels to the same first-hop

When we have many channels to the same first-hop, many of which do
not have sufficient balance to make the requested payment, but when
some do, instead of simply using the available channel balance we
may switch to MPP, potentially with many, many paths.

Instead, we should seek to use the smallest channel which can
easily handle the requested payment, which we do here by sorting
the first_hops in our router before beginning the graph search.

Note that the "real" fix for this should be to instead decide which
channel to use at HTLC-send time, as most other nodes do during
relay, but this provides a minimal fix without needing to do the
rather-large work of refactoring our HTLC send+relay pipelines.

Issues with overly-aggressive MPP on many channels were reported by
Cash App.