Added a request logs file

[douxxtech]

This PR adds a LOG_FILE config key, empty by default. If a path to a file is set, the request logs (only the common log format requests logs) will be appended to the given file instead of being outputed to STDOUT

[github-actions]

START + GET /: 150 syscalls
Fastest: 0.000015s (write)
Slowest: 0.861084s (accept)

threads: 4 (auto), conns: 50, duration: 20s

[wrk]
Running 20s test @ http://127.0.0.1:8080/
  4 threads and 50 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     5.25ms    8.09ms 213.64ms   99.45%
    Req/Sec     1.52k   340.88     2.35k    69.00%
  121099 requests in 20.06s, 185.36MB read
  Socket errors: connect 0, read 2853, write 0, timeout 0
Requests/sec:   6038.27
Transfer/sec:      9.24MB

[github-actions]

START + GET /: 150 syscalls
Fastest: 0.000017s (clock_gettime)
Slowest: 0.811086s (accept)

threads: 4 (auto), conns: 50, duration: 20s

[wrk]
Running 20s test @ http://127.0.0.1:8080/
  4 threads and 50 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     5.34ms    9.04ms 214.17ms   99.47%
    Req/Sec     1.54k   317.54     2.29k    71.62%
  122989 requests in 20.04s, 188.25MB read
  Socket errors: connect 0, read 252, write 0, timeout 0
Requests/sec:   6138.50
Transfer/sec:      9.40MB