multi: add NewLogClosure in lnutils to avoid repetition

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

discovery/bootstrapper.go

@@ -16,6 +16,7 @@ import (
 	"github.com/btcsuite/btcd/btcutil/bech32"
 	"github.com/davecgh/go-spew/spew"
 	"github.com/lightningnetwork/lnd/autopilot"
+	"github.com/lightningnetwork/lnd/lnutils"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/tor"
 	"github.com/miekg/dns"
@@ -431,7 +432,7 @@ search:
 		}
 
 		log.Tracef("Retrieved SRV records from dns seed: %v",
-			newLogClosure(func() string {
+			lnutils.NewLogClosure(func() string {
 				return spew.Sdump(addrs)
 			}),
 		)

discovery/log.go

@@ -27,19 +27,3 @@ func DisableLog() {
 func UseLogger(logger btclog.Logger) {
 	log = 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
-
-// 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)
-}