switch to case-insensitive http header comparisons, also fix racing in linux.jai

[DaseinPhaos]

Thanks for the awesome package!

This PR fixes a tiny issue wrt http header comparisons. According to the mdn reference:

An HTTP header consists of its case-insensitive name followed by a colon...

But the previous parsing code used hard-coded literals when parsing specific headers, resulting in some requests not being correctly parsed. This PR fixes that.

[DaseinPhaos]

Also, the crashing issue you mentioned might be related to the racing issue when accessing thread_pool: the built-in Hash_Table is not thread safe and thus each pool thread should be assigned its own hash table to avoid racing. This PR also did that.

However, I don't have a beefy ubuntu server with a lot of threads to properly test it.

Testing with THREAD_POOL_COUNT=2 and wrk --latency -t6 -c10000 -d1 http://localhost/bench results in the following result, which I'm not sure counts as a successful fix or not:

Running 1s test @ http://localhost/bench
  6 threads and 10000 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency    18.74ms   64.24ms 662.23ms   91.66%
    Req/Sec     9.16k     5.14k   28.67k    74.00%
  Latency Distribution
     50%   27.00us
     75%   38.00us
     90%   17.72ms
     99%  356.75ms
  47328 requests in 1.10s, 4.38MB read
  Socket errors: connect 8981, read 0, write 0, timeout 0
  Non-2xx or 3xx responses: 47328
Requests/sec:  42991.22
Transfer/sec:      3.98MB

But it's certainly not crashing anymore!