Adds liquidity hints for the sending capabilities of routing channels in the
graph. The channel blacklist is incorporated into liquidity hints.
Liquidity hints are updated when a payment fails with a temporary
channel failure or when it succeeds. Liquidity hints are used to give a
penalty in the _edge_cost heuristics used by the pathfinding algorithm.
The base penalty in (_edge_cost) is removed because it is now part of the
liquidity penalty. We don't return early from get_distances, as we want
to explore all channels.
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