discovery: add AssumeChannelValid config option

in preparation for later on when we need to know when to skip funding
transaction validation.

discovery/gossiper.go

@@ -364,6 +364,11 @@ type Config struct {
 	// updates for a channel and returns true if the channel should be
 	// considered a zombie based on these timestamps.
 	IsStillZombieChannel func(time.Time, time.Time) bool
+
+	// AssumeChannelValid toggles whether the gossiper will check for
+	// spent-ness of channel outpoints. For neutrino, this saves long
+	// rescans from blocking initial usage of the daemon.
+	AssumeChannelValid bool
 }
 
 // processedNetworkMsg is a wrapper around networkMsg and a boolean. It is