httputil: Fix quadratic performance of cookie parsing

httputil: Fix quadratic performance of cookie parsing

Maliciously-crafted cookies can cause Tornado to
spend an unreasonable amount of CPU time and block
the event loop.

This change replaces the quadratic algorithm with
a more efficient one. The implementation is copied
from the Python 3.13 standard library (the
previous one was from Python 3.5).

Fixes CVE-2024-52804
See CVE-2024-7592 for a similar vulnerability in cpython.

Thanks to github.com/kexinoh for the report.