PERF: use buffer pool in json formatter

[taylorchu]

benchmark             old ns/op     new ns/op     delta
BenchmarkLogrus-8     4163          4369          +4.95%

benchmark             old allocs     new allocs     delta
BenchmarkLogrus-8     36             31             -13.89%

benchmark             old bytes     new bytes     delta
BenchmarkLogrus-8     3027          2163          -28.54%

func BenchmarkLogrus(b *testing.B) {
	log := logrus.New()
	log.Formatter = &logrus.JSONFormatter{}
	log.Out = ioutil.Discard
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		log.
			WithField("foo1", "bar1").
			WithField("foo2", "bar2").
			Info("this is a dummy log")
	}
}

[taylorchu]

@sirupsen this idea is also in text_formatter. Could you take a look? thanks!

[sirupsen]

@stevvooe @seiffert do you mind taking a look at this?