feat: refactoring CgiHandler

[StokesAsselborn]

Main changes :

1. Refactoring of the CgiHandler
2. Tweaking the client.cpp for the refresh_interest_list(), write_to_virtual_file() and read_from_virtual_file()

Make test = ok

Specific curls :

Test OK
curl -i -G "http://localhost:8080/cgi-py/cgi_upper.py"   --data-urlencode "hello=$(python3 - <<'PY'
print("a"*666)
PY





)"

Test NOK
curl -i -G "http://localhost:8080/cgi-py/cgi_upper.py"   --data-urlencode "hello=$(python3 - <<'PY'
print("a"*666666)
PY





)"

Test OK but only once. Rerunning the command yields a broken pipe as the epoll list is not properly managed
curl -i -X POST "http://localhost:8080/cgi-py/cgi_upper.py"   -H "Content-Type: text/plain"   --data "hello from post"

[StokesAsselborn]

I pushed a working version of the refactored CgiHandler.
I let you judge which changes in the client.cpp need to be considered.

[iboukhss]

OK so I merged #109 just we have a stable base to work on. This PR tho I cannot merge right now. It's too hacky.

I will first push my fix for UploadHandler and then I can help you reconcile this.

We shouldn't have to change the client and server code that much to make things work.

The most problematic functions that are most likely buggy are Server::update_interest_list and Client::refresh_interest_list, too many if/else, everything else should be solid and simple to review.