multi: add NewLogClosure in lnutils to avoid repetition

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

watchtower/wtdb/log.go

@@ -43,19 +43,3 @@ func UseLogger(logger btclog.Logger) {
 	migration7.UseLogger(logger)
 	migration8.UseLogger(logger)
 }
-
-// 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 // nolint:unused
-
-// 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 { // nolint:unused
-	return logClosure(c)
-}