Add router support for batched splices

[remyers]

A splice tx could involve more than one parent channel. The Router must track the set of channels spent by a given tx until matching channel announcements are received that splice the parent channels or the spend tx is deeply buried.

If a splice tx spends more than one parent channel between the same nodes, then there's no way to know which new channel announcement corresponds to which parent channel. We simply update the first one found.

[t-bast]

This was more complex than I expected, I hadn't initially thought about the fact that we cannot map a spliced channel to its parent (but it's fine we don't care about which parent we map it to).

[t-bast]

Needs a rebase, almost there!

[remyers]

Needs a rebase, almost there!

Thanks for the review! Rebased on master.

I also added some new checks in the gossip integration test to check the state of the channels set in the Router after the splice. I found it included the original parent channel when it should have only had the splice_ab and bc channels. The fix was to wait for Alice to receive both initial local channel updates before doing the splice. Otherwise the 2nd update (from Bob?) after opening the original ab channel came after the first splice update and added the original channel as a new channel in the Router.

[t-bast]

A few test nits, otherwise LGTM.