We defer the notification to gossipd till the end of the spends. By itself not
a huge change, but it allows us to later migrate to doing updates blindly and
using the DB as our ground truth. It also allows us to simplify the
`wallet_outpoint_spend` semantics to not return two values in the next commit.
We're going to want this for bolt13 formation as well.
As a result of reworking the logic into "candidate selection" then
"route hint selection", we need to change the way round-robin works.
We use a simple incrementing index now.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
