channeldb: don't use iota for the ChannelType enum

This commit modifies the ChannelType enum to no longer use iota as
changes in the definition would cause the values to shift, breaking the
long-term stability required for persistence. Instead, we now select
values manually to indicate the particular channel type.

channeldb/channel.go

@@ -103,14 +103,17 @@ var (
 type ChannelType uint8
 
 const (
+	// NOTE: iota isn't used here for this enum needs to be stable
+	// long-term as it will be persisted to the database.
+
 	// SingleFunder represents a channel wherein one party solely funds the
 	// entire capacity of the channel.
-	SingleFunder = iota
+	SingleFunder = 0
 
 	// DualFunder represents a channel wherein both parties contribute
 	// funds towards the total capacity of the channel. The channel may be
 	// funded symmetrically or asymmetrically.
-	DualFunder
+	DualFunder = 1
 )
 
 // OpenChannel encapsulates the persistent and dynamic state of an open channel