Efficient string concatenation

[catenacyber]

Found by oss-fuzz
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=21892

The optimization comes from
https://hermanschaaf.com/efficient-string-concatenation-in-go/

My benchmark on the oss-fuzz test case is from 80 seconds to 0.1 second with this PR

Reproducer program is

package main

import (
	"fmt"
	"io/ioutil"
	"os"
	"runtime/pprof"

	"github.com/miekg/dns"
)

func main() {
	f, err := os.Create("cpu.prof")
	if err != nil {
		fmt.Printf("error %#+v\n", err)
		return
	}
	_ = pprof.StartCPUProfile(f)

	data, err := ioutil.ReadFile(os.Args[1])
	if err != nil {
		fmt.Printf("Failed to read corpus file", err)
	}
	fmt.Printf("Starting %s\n", os.Args[1])
	dns.NewRR(string(data))

	fmt.Printf("Finished\n")
	pprof.StopCPUProfile()
	f, err = os.Create("heap.prof")
	if err != nil {
		fmt.Printf("error %#+v\n", err)
		return
	}
	if err = pprof.WriteHeapProfile(f); err != nil {
		fmt.Printf("error %#+v\n", err)
		return
	}
	f.Close()
	fmt.Printf("end\n")
}

[codecov-io]

Codecov Report

Merging #1105 into master will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1105      +/-   ##
==========================================
+ Coverage   55.61%   55.66%   +0.04%     
==========================================
  Files          41       41              
  Lines       10082    10082              
==========================================
+ Hits         5607     5612       +5     
+ Misses       3445     3441       -4     
+ Partials     1030     1029       -1     

Impacted Files	Coverage Δ
scan_rr.go	60.82% <100.00%> (ø)
server.go	70.71% <0.00%> (+0.27%)	⬆️
msg.go	77.36% <0.00%> (+0.67%)	⬆️

---

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