Reduce slice growth in adaptor

[moycat]

This PR improves the performance of fasthttpadaptor by reducing the overhead of slice growth.

The current adapter records all written body in a byte slice, which causes horrible overhead when the body is bigger than a few kilobytes. However, this is unnecessary as we can just pass the response writer of fasthttp.

fasthttp/fasthttpadaptor/adaptor.go

Lines 108 to 111 in ea60524

	func (w *netHTTPResponseWriter) Write(p []byte) (int, error) {
	w.body = append(w.body, p...)
	return len(p), nil
	}

I compared the performance by modifying the ServerGet tests with 32 KiB responses.

The original ServerGet without the adaptor:

BenchmarkServerGet1ReqPerConn-4                          4658205              2622 ns/op               0 B/op          0 allocs/op
BenchmarkServerGet2ReqPerConn-4                          5202049              2466 ns/op               0 B/op          0 allocs/op
BenchmarkServerGet10ReqPerConn-4                         7114965              1754 ns/op               0 B/op          0 allocs/op
BenchmarkServerGet10KReqPerConn-4                        8304604              1486 ns/op               0 B/op          0 allocs/op
BenchmarkServerGet1ReqPerConn10KClients-4                4690203              2395 ns/op              12 B/op          0 allocs/op
BenchmarkServerGet2ReqPerConn10KClients-4                5133776              2260 ns/op               3 B/op          0 allocs/op
BenchmarkServerGet10ReqPerConn10KClients-4               7322839              1583 ns/op               1 B/op          0 allocs/op
BenchmarkServerGet100ReqPerConn10KClients-4              8498665              1402 ns/op               2 B/op          0 allocs/op

The ServerGet with the current adaptor:

BenchmarkAdaptorGet1ReqPerConn-4                 1000000             14145 ns/op           36900 B/op         33 allocs/op
BenchmarkAdaptorGet2ReqPerConn-4                  819870             13954 ns/op           36749 B/op         32 allocs/op
BenchmarkAdaptorGet10ReqPerConn-4                 932516             11769 ns/op           35295 B/op         30 allocs/op
BenchmarkAdaptorGet10KReqPerConn-4               1116939             10816 ns/op           34562 B/op         30 allocs/op
BenchmarkAdaptorGet1ReqPerConn10KClients-4        558528             24187 ns/op           46985 B/op         44 allocs/op
BenchmarkAdaptorGet2ReqPerConn10KClients-4        745404             14326 ns/op           40126 B/op         35 allocs/op
BenchmarkAdaptorGet10ReqPerConn10KClients-4       920809             10949 ns/op           36740 B/op         32 allocs/op
BenchmarkAdaptorGet100ReqPerConn10KClients-4     1320006              8900 ns/op           34945 B/op         30 allocs/op

The ServerGet with the patched adaptor:

BenchmarkAdaptorGet1ReqPerConn-4                 3693327              3372 ns/op            1911 B/op         30 allocs/op
BenchmarkAdaptorGet2ReqPerConn-4                 3534920              3360 ns/op            1887 B/op         29 allocs/op
BenchmarkAdaptorGet10ReqPerConn-4                4373206              2716 ns/op            1893 B/op         29 allocs/op
BenchmarkAdaptorGet10KReqPerConn-4               4955851              2424 ns/op            1776 B/op         29 allocs/op
BenchmarkAdaptorGet1ReqPerConn10KClients-4       2716551              4831 ns/op            3189 B/op         31 allocs/op
BenchmarkAdaptorGet2ReqPerConn10KClients-4       3118240              4085 ns/op            2953 B/op         30 allocs/op
BenchmarkAdaptorGet10ReqPerConn10KClients-4      4207117              3088 ns/op            2383 B/op         29 allocs/op
BenchmarkAdaptorGet100ReqPerConn10KClients-4     5002465              2379 ns/op            1926 B/op         29 allocs/op

[erikdubbelboer]

Thanks!