We used to agree up on the `minimum_depth` with the peer, thus when
they told us that the funding locked we'd be sure we either have a
scid or we'd trigger the state transition when we do. However if we
had a scid, and we got a funding_locked we'd trust them not to have
sent it early. Now we explicitly track the depth in the channel while
waiting for the funding to confirm.
Changelog-Fixed: channeld: Enforce our own `minimum_depth` beyond just confirming
