Use slice to build domain string for unpacking

Considerably faster than simply using string concatenation. A lot more memory efficient too.

```
BenchmarkMsgUnpack	   50000	     57474 ns/op	   16730 B/op	     674 allocs/op
```

```
BenchmarkMsgUnpack	  500000	      7247 ns/op	     813 B/op	      23 allocs/op
``` 

I also added a benchmark to monitor domain unpacking performance in the future.