We had one report of this, and then Eugene and Roasbeef of Lightning
Labs confirmed it; they saw misordered HTLCs on reconnection too.
Since we didn't enforce this when we receive HTLCs, we never noticed :(
Fixes: #3920
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Fixed: Protocol: fixed retransmission order of multiple new HTLCs (causing channel close with LND)