In this commit, we thread through the necessary state to allow users to
set a max shard amount. If this value is set, then this'll effectively
serve as a ceiling for all our split attempts. If we need to split,
we'll first try to use `paymentAmt/2`, if that's bigger than
`MaxShardAmt, then we'll use the latter instead.
Ideally in the future we have a dynamic way to automatically set both
the `MaxShardAmt` as well as `MaxParts` for users. Until then exposing
these two new fields will allow us to experiment with setting them
automatically using the RPC interface, and also give users a bit more
control over how we attempt to route payments, akin to coin control for
on-chain payments.
Fixes#4730
b73a6e2c61