utxonursery: pass broadcastHeight to registerPreschoolConf

In this commit IncubateOutputs are given an extra parameter
broadcastHeight, which is passed from the server and used when called
registerPresschoolConf.

Earlier the utxonursery used the bestHeight as height hint in this case,
which would be wrong in the cases where we got outputs for incubation
which was confirmed below the best height.

server.go

@@ -644,6 +644,7 @@ func newServer(listenAddrs []net.Addr, chanDB *channeldb.DB, cc *chainControl,
 
 			return s.utxoNursery.IncubateOutputs(
 				chanPoint, commitRes, outRes, inRes,
+				broadcastHeight,
 			)
 		},
 		PreimageDB:   s.witnessBeacon,