peer+htlcswitch: update Enable/DisableAdds API

In this commit, the `ChannelUpdateHandler`'s `EnableAdds` and
`DisableAdds` methods are adjusted to return booleans instead of errors.
This is done becuase currently, any error returned by these methods is
treated by just logging the error since today all it means is that the
proposed update has already been done. And so all we do today is log the
error. But in future, if these methods are updated to return actual
errors that need to be handled, then we might forget to handle them
correctly at the various call sights. So we instead change the signature
of the function to just return a boolean. In future, if we do need to
return any error, we will have to go inspect every call sight in any
case to fix compliation & then we can be sure we are handling the errors
correctly.

htlcswitch/interfaces.go

@@ -135,14 +135,16 @@ type ChannelUpdateHandler interface {
 	MayAddOutgoingHtlc(lnwire.MilliSatoshi) error
 
 	// EnableAdds sets the ChannelUpdateHandler state to allow
-	// UpdateAddHtlc's in the specified direction. It returns an error if
-	// the state already allowed those adds.
-	EnableAdds(direction LinkDirection) error
+	// UpdateAddHtlc's in the specified direction. It returns true if the
+	// state was changed and false if the desired state was already set
+	// before the method was called.
+	EnableAdds(direction LinkDirection) bool
 
 	// DisableAdds sets the ChannelUpdateHandler state to allow
-	// UpdateAddHtlc's in the specified direction. It returns an error if
-	// the state already disallowed those adds.
-	DisableAdds(direction LinkDirection) error
+	// UpdateAddHtlc's in the specified direction. It returns true if the
+	// state was changed and false if the desired state was already set
+	// before the method was called.
+	DisableAdds(direction LinkDirection) bool
 
 	// IsFlushing returns true when UpdateAddHtlc's are disabled in the
 	// direction of the argument.