multi: add NewLogClosure in lnutils to avoid repetition

And replaces all usage of `logClosure` with `lnutils.LogClosure`.

lnutils/log.go

@@ -0,0 +1,17 @@
+package lnutils
+
+// LogClosure is used to provide a closure over expensive logging operations so
+// don't have to be performed when the logging level doesn't warrant it.
+type LogClosure func() string
+
+// String invokes the underlying function and returns the result.
+func (c LogClosure) String() string {
+	return c()
+}
+
+// NewLogClosure returns a new closure over a function that returns a string
+// which itself provides a Stringer interface so that it can be used with the
+// logging system.
+func NewLogClosure(c func() string) LogClosure {
+	return LogClosure(c)
+}