[WIP] Some optimization

- don't use strftime()
- don't copy pointer for readability
- remove debug output
- optimize check_path
- don't set 200 status code per request and implement mrb_http2_200_send_response
- use fstat() instead of stat()

#### before
```
$ h2load -v -c 500 -m 100 -n 2000000 http://127.0.0.1:8081/index.html
starting benchmark...
spawning thread #0: 500 concurrent clients, 2000000 total requests

finished in 11 sec, 435 millisec and 336 microsec, 174896 req/s, 8383 kbytes/s
requests: 2000000 total, 2000000 started, 2000000 done, 2000000 succeeded, 0 failed, 0 errored
status codes: 2000000 2xx, 0 3xx, 0 4xx, 0 5xx
traffic: 98170881 bytes total, 18158881 bytes headers, 44000000 bytes data
```

#### after
```
$ h2load -v -c 500 -m 100 -n 2000000 http://127.0.0.1:8081/index.html
starting benchmark...
spawning thread #0: 500 concurrent clients, 2000000 total requests

finished in 10 sec, 254 millisec and 765 microsec, 195031 req/s, 9347 kbytes/s
requests: 2000000 total, 2000000 started, 2000000 done, 2000000 succeeded, 0 failed, 0 errored
status codes: 2000000 2xx, 0 3xx, 0 4xx, 0 5xx
traffic: 98156000 bytes total, 18144000 bytes headers, 44000000 bytes data
```