[FEATURE REQUEST] Support multi path payments

[flexo82]

Since LND v0.9.0 multi path payments has been possible. If rebalances could utilize this it would be easier to rebalance those "hard-to-balance" channels or do larger rebalances since this could be spread over multiple paths

[C-Otto]

I've thought about this, more than once. I believe rebalance transactions don't need to be atomic, and I also believe that sending a fraction of the intended amount is better than not doing anything. Based on this, why don't you just try again with smaller amounts? Instead of, say, sending 1M sat on one go, just try 100k sat up to 10 times?

I'm very interested in learning about the benefits of MPPs.