net/http: lack of limit when parsing cookies can cause memory exhaustion

Despite HTTP headers having a default limit of 1 MB, the number of cookies that can be parsed did not have a limit.
By sending a lot of very small cookies such as "a=;", an attacker can make an HTTP server allocate a large amount of structs, causing large memory consumption.

net/http now limits the number of cookies accepted to 3000, which can be adjusted using the httpcookiemaxnum GODEBUG option.