There is a race when initiating multiple lightning payments concurrently
(e.g. when doing a reverse swap with prepayment + swap payment).
suggest_splits might overallocate
split amounts for a channel as the splitting of both invoice amounts runs
concurrently and before acutal htlcs that reduce the channels balance
have been added to the channel yet. This results in a "not
enough balance" PaymentFailure once we try to send the htlcs and the
other payment attempt already reduced the available balance of the
channel.
This fix takes a lock from splitting the amount until the htlcs are
put on the channel, so suggest_splits always acts on the correct channel
balance.
196cc33c1c