We need to know when changes are fully committed by both sides:
1) For their HTLC_ADDs, this is when we can fulfill/fail/route.
2) For their HTLC_FAILs, this is when we can fail incoming.
For HTLC_FULFULL we don't need to wait: as soon as we know the preimage
we can propogate it.
For the moment, we simply log and assert; acting on it comes later.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>