channeld: send warning, not error if peer has old commitment number.

[rustyrussell]

trafficstars

This is the minimal change to meet the desired outcome of https://github.com/lightning/bolts/issues/934 which wants to give obsolete-db nodes a chance to fix things up, before we close the channel.

We need to dance around a bit here, since we will close the channel if we receive an ERROR, so we suppress that.

[t-bast]

This title of the PR is misleading, shouldn't it be exactly the opposite (ie send warning, not error)?

[cdecker]

Rebased on top of master to get CI to rerun and ready the PR for merge.