chore: improve /healthz endpoint performance (#2014)

* Add benchmark for /healthz endpoint

This benchmark will measure not only speed but also how much memory
each request to /healthz allocate. I'm adding the benchmark to have a
baseline to compare.

Signed-off-by: Leandro López (inkel) <inkel.ar@gmail.com>

* Hardcode /healthz body into a variable

We know it will always be this value, there's no need to import and
call json.MarshalIndent.

Signed-off-by: Leandro López (inkel) <inkel.ar@gmail.com>

* Initialize /healthz body only once

Signed-off-by: Leandro López (inkel) <inkel.ar@gmail.com>

server/server_test.go

@@ -147,6 +147,25 @@ func TestHealthz(t *testing.T) {
 }`, string(body))
 }
 
+type mockRW struct{}
+
+var _ http.ResponseWriter = mockRW{}
+var mh = http.Header{}
+
+func (w mockRW) WriteHeader(int)           {}
+func (w mockRW) Write([]byte) (int, error) { return 0, nil }
+func (w mockRW) Header() http.Header       { return mh }
+
+var w = mockRW{}
+var s = &server.Server{}
+
+func BenchmarkHealthz(b *testing.B) {
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		s.Healthz(w, nil)
+	}
+}
+
 func TestParseAtlantisURL(t *testing.T) {
 	cases := []struct {
 		In     string