refactor(*) proxy-wasm runloop + req state enhancements

[thibaultcha]

Improvements to the proxy-wasm runloop extracted from eof flag work.

[codecov]

Codecov Report

Merging #508 (e3f2bd8) into main (45a0c12) will decrease coverage by 0.00707%.
The diff coverage is 100.00000%.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main        #508         +/-   ##
===================================================
- Coverage   90.02022%   90.01315%   -0.00707%     
===================================================
  Files             46          46                 
  Lines           9890        9883          -7     
===================================================
- Hits            8903        8896          -7     
  Misses           987         987                 

Files	Coverage Δ
src/common/ngx_wasm_socket_tcp.c	88.82759% <100.00000%> (ø)
src/common/ngx_wasm_subsystem.c	100.00000% <ø> (ø)
src/common/proxy_wasm/ngx_proxy_wasm.c	92.84091% <100.00000%> (+0.04090%)	⬆️
src/common/proxy_wasm/ngx_proxy_wasm.h	91.89189% <ø> (ø)
src/common/proxy_wasm/ngx_proxy_wasm_host.c	94.29348% <100.00000%> (ø)
src/http/ngx_http_wasm_filter_module.c	92.73743% <100.00000%> (-0.15874%)	⬇️
src/http/ngx_http_wasm_module.c	96.00939% <100.00000%> (ø)
src/http/proxy_wasm/ngx_http_proxy_wasm.c	93.51351% <100.00000%> (-0.06939%)	⬇️
src/http/proxy_wasm/ngx_http_proxy_wasm_dispatch.c	91.22807% <100.00000%> (ø)
src/wasm/ngx_wasm.h	100.00000% <ø> (ø)
... and 1 more

Flag	Coverage Δ
unit	90.07941% <100.00000%> (-0.00458%)	⬇️
valgrind	77.84173% <100.00000%> (-0.02206%)	⬇️

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