Put the ModSecurity interception logic onto separated threads

[wfjsw]

This PR is created to describe my patch to fix #227 and it is by no means a complete patch ready for merge.

The patch contains several unrelated changes, namely:

• Logger change, from dd to ngx_log_error to accomodate my own debugging need
• Removal of ngx_http_modsecurity_pcre_malloc_init and ngx_http_modsecurity_pcre_malloc_done. They are not used in my configuration where PCRE2 is used, and it looks suspicious for SEGVs so I commented them out as a precaution.
• Why use NGX_OK in logging handler? Changed to NGX_DECLINED.

Not yet implemented:

• I'm not sure whether logging phase would take advantage of this. I did not change that yet.
• Missing a NGX_THREADS guard for Nginx setup without threading support.

Currently it passes all test suites and performs well in production.

Benchmarking is welcomed.

[airween]

Hi @wfjsw, there is a new CI workflow test in this repository. Could you pick up the modifications to enable run those tests? Thanks!

[jeremyjpj0916]

@airween @wfjsw I wonder if multi threading is at least the partial answer to fix the long known problem of ModSecurity+nginx being a performance killer for throughput.

[wfjsw]

I no longer have a ModSecurity install on my machine so I'm unable to investigate further :(

Re performance issue: From the CPU loads it still seems heavy. I'd say there is real computing constraint in WAF. (Or it might be a problem caused by PCRE2. Who knows)