Use strings builder for attester lock key

[terencechain]

What type of PR is this?

Minor improvement

What does this PR do? Why is it needed?

Use strings builder to construct lock key for attesting. It is more performant than casting. It's about 70% faster and 10% less bytes per allocation.

BenchmarkTwoInputs-12    	 2901903	       397 ns/op	     432 B/op	       7 allocs/op
BenchmarkOneInputWithConcat-12    	 3710899	       316 ns/op	     448 B/op	       7 allocs/op
BenchmarkOneInputWithStringBuilder-12    	 4719397	       245 ns/op	     408 B/op	       7 allocs/op


func BenchmarkTwoInputs(b *testing.B) {
	pubKey := [48]byte{'a'}
	b.ReportAllocs()
	for i := 0; i < b.N; i++ {
		mputil.NewMultilock(fmt.Sprint(roleAttester), string(pubKey[:]))
	}
}

func BenchmarkOneInputWithConcat(b *testing.B) {
	pubKey := [48]byte{'a'}
	b.ReportAllocs()
	for i := 0; i < b.N; i++ {
		mputil.NewMultilock(fmt.Sprint(roleAttester) + string(pubKey[:]))
	}
}

func BenchmarkOneInputWithStringBuilder(b *testing.B) {
	pubKey := [48]byte{'a'}
	b.ReportAllocs()
	for i := 0; i < b.N; i++ {
		var b strings.Builder
		b.WriteByte(byte(roleAttester))
		b.Write(pubKey[:])
		mputil.NewMultilock(b.String())
	}
}

Which issues(s) does this PR fix?

N/A

Other notes for review
Leaving proposing side untouched because it's less relevant