Be willing to pick smaller paths at the end of pathfinding if there's not much left to find

[TheBlueMatt]

If we have only marginally more available to send than we're trying to send, and pick a few MPP paths which total just less than we're trying to send, we might find ourselves stuck as we won't be willing to pick a small path to finish out the rest if what we need is < 1/max_path_count (default 10).

eg if we have 2.6M sats available to send, and are trying to send 2.5, if we get unlucky and the paths we pick total to 2.4, we will refuse to pick another path because our first hop channel doesn't have enough to meet our path minimum (2.5M/10 = 250k) but we should really be willing to relax that minimum because we only need another 100k sats left.