Optimize websocket headers handling

[juliens]

What does this PR do?

Improves performance for non websocket upgrade requests.

Motivation

Improve performance !!!!!!

More

- [ ] Added/updated tests
- [ ] Added/updated documentation

Additional Notes

Before

Running 30s test @ http://traefik/bench
  4 threads and 100 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     1.63ms    1.11ms  44.00ms   81.79%
    Req/Sec    16.22k     1.23k   54.86k    99.17%
  Latency Distribution
     50%    1.42ms
     75%    2.07ms
     90%    2.87ms
     99%    5.03ms
  1938441 requests in 30.10s, 188.56MB read
Requests/sec:  64401.47
Transfer/sec:      6.26MB

After

Running 30s test @ http://traefik/bench
  4 threads and 100 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     1.56ms    1.04ms  53.60ms   81.83%
    Req/Sec    16.93k   532.62    18.78k    93.25%
  Latency Distribution
     50%    1.36ms
     75%    1.97ms
     90%    2.71ms
     99%    4.79ms
  2021318 requests in 30.02s, 196.62MB read
Requests/sec:  67331.54
Transfer/sec:      6.55MB

Co-authored-by: Kevin Pollet pollet.kevin@gmail.com

[ldez]

LGTM

[kevinpollet]

LGTM 🚀

[rtribotte]

LGTM 👍