YaHTTP: Enforce max # of request fields and max request line size

YaHTTP: Enforce max # of request fields and max request line size

The default values, 8192 bytes for the maximum request line size and
100 fields, are taken from the default settings of Apache HTTPd:
- https://httpd.apache.org/docs/2.2/mod/core.html#limitrequestline
- https://httpd.apache.org/docs/2.2/mod/core.html#limitrequestfields

Reported by OSS-Fuzz as a timeout in https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=67993

(cherry picked from commit 249c86a63865e4a980511839887890c3940fc6e6)