Previously, the expiration timer for learned session was not started
until a new session is created.  This could lead to the sessions being
active past the expiration time.

Adds new options for the "sticky cookie" directive to set
samesite=<strict|lax|none> cookie flags.

With this parameter set, sessions are learned after receiving upstream headers.

RFC 6265 defines "Max-Age" cookie attribute in section 5.2.2.

If the "expires" option is passed to the "sticky" directive, "max-age"
attribute will appear in cookies set by the module with corresponding
value in seconds.

For the special "max" value of the "expires" option, corresponding "max-age"
attribute value will be set to 315360000 seconds (10 years, similar to
how its done in headers_filter module for the "Cache-Control" header).

The attributes are described in RFC6265, sections 4.1.2.5 and 4.1.2.6
respectively.

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>

Sticky sessions allow to route the same client to the same upstream server.

- upstream structures are extended to keep session-related information

- existing balancing modules are updated to provide an id of the selected
  server (SID) in pc->sid, and to select the server, given it's SID.

- other balancing modules are allowed to set the pc->hint value to choose
  the desired peer.  The sticky module will not change the hint if it's
  already set.

- the feature is enabled by default and can be disabled with the
  "--without-http_upstream_sticky" switch of the configure script.

The following configuration can be used to enable sticky sessions for
supported balancing modules:

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

        sticky cookie server_id expires=1h domain=.example.com path=/;
    }

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