[feature]: `SendPayment`: allow setting`max_shard_size_asset`

[ZZiigguurraatt]

With #1613, I'm not sure how it decides to automatically shard assets when using SendPayment over multiple outbound channels. When manually sharding assets, I'm not sure which quote is used with the max_shard_size_msat. Does it use the highest quote, the lowest quote, or the quote that goes with that channel? Regardless of how it does it, it is difficult to choose a good shard size when we don't know what any of the quotes are in advance. I'd like an option max_shard_size_asset that lets us set the shard size in terms of the asset being sent and not msat, this way the user has better control over what is going on.

[Roasbeef]

Just like in normal LN, we don't really expect users to ever really need to set the max shard size for routine payments. The path finding algorithm is greedy, in that it'll only attempt to split when it really needs to. It decides to split when the internal path finding probability estimator returns a value below our min prob attempt value.

A user that worries about the individual size of a payment shard is a fairly advanced user and knows a good bit about what's going on under the hood.

[Roasbeef]

When manually sharding assets, I'm not sure which quote is used with the max_shard_size_msat

I think the answer to that is: a quote isn't necessarily used along with that field. Instead only once lnd obtains the sat amount to spend, is the payment subject to splitting, based on that max_shard_size_msat value. By default, lnd will split that amount by two, then try to independent sends with that new amount.

[ZZiigguurraatt]

OK, so you want to close this as wontfix?