Sticky: added the "learn" mode.

Sticky: added the "learn" mode.

In this mode, nginx "learns" which client uses which proxied server by
analyzing headers of client requests and proxied server responses.

For example, a proxied server may start sessions by issuing the "Set-Cookie"
header field to set cookie 'sid' and returning clients will bring the cookie
with the same name.

The following configuration may be used to handle this case:

upstream u1 {
    server 127.0.0.1:8080;
    server 127.0.0.1:8081;

    sticky learn timeout=10m zone=sess:1m
           create=$upstream_cookie_sid
           lookup=$cookie_sid;
}

Co-authored-by: Ruslan Ermilov <ru@nginx.com>
Co-authored-by: Maxim Dounin <mdounin@mdounin.ru>