rebalance: Allow "triangle" rebalances or arbitrary route specification

[ajpwahqgbi]

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Liquidity swap triangles are increasingly popular, and for newly-created triangles it may make sense to rebalance specifically along the triangle. That's not easy to do with the current implementation.

Current behavior:

Rebalancing chooses routes according to its own ideas, and there is no way to specify a route to attempt.

Desired behavior:

Either:

1. The ability to specifically do a triangle rebalance. This would not require any changes to the command line arguments; it just needs restricting path selection to those channels that directly connect the outgoing and incoming channel peers.
2. The ability to specify (by JSON array of pubkeys?) a route (or just the mid_route portion?) to attempt the rebalance along.

[manreo]

I did not get 1. but for 2. you can just use lightning-cli sendpay route, not need for the rebalance plugin