Use 48-bit commitment transaction numbers

Fix SetStateNumHint and GetStateNumHint to properly
set and get the stateNumHints using the lower 24 bits
of the locktime of the commitment transaction as the
lower 24 bits of the obfuscated state number and the
lower 24 bits of the sequence field as the higher 24
bits.

channeldb/channel.go

@@ -185,7 +185,7 @@ type OpenChannel struct {
 	// StateHintObsfucator are the btyes selected by the initiator (derived
 	// from their shachain root) to obsfucate the state-hint encoded within
 	// the commitment transaction.
-	StateHintObsfucator [4]byte
+	StateHintObsfucator [6]byte
 
 	// ChanType denotes which type of channel this is.
 	ChanType ChannelType