Need help with sliding sessions on SharePoint 2013

Question

My setup uses claims based authentication with an SQL membership provider.
I'm trying to get sliding expiration to work.  
I added the the following method in global.asax file  based on http://michael-mckenna.com/blog/the-problem-with-absolute-token-expiration-in-windows-identity-foundation-wif
protected void SessionAuthenticationModule_SessionSecurityTokenReceived(object sender, SessionSecurityTokenReceivedEventArgs e)
DateTime now = DateTime.UtcNow;
DateTime validFrom = e.SessionToken.ValidFrom;
DateTime validTo = e.SessionToken.ValidTo;
if ((now < validTo) && (now > validFrom.AddMinutes((validTo.Minute - validFrom.Minute) / 2)))
SessionAuthenticationModule sam = sender as SessionAuthenticationModule;
e.SessionToken = sam.CreateSessionSecurityToken(e.SessionToken.ClaimsPrincipal, e.SessionToken.Context,
now, now.AddMinutes(SLIDING_TIMEOUT_VALUE), e.SessionToken.IsPersistent);
e.ReissueCookie = true;
I used Fiddler to monitor the connection and see a new FedAuth cookie sent to the browser whenever the "if" condition is satisfied.  The problem is that the session still times out.  When I compared the value of re-issued FedAuth to that
of the earlier FedAuth, the values were identical even though the times for ValidFrom and ValidTo of the session token should be different.
Why does the value not change?  What am I missing?
Update: According to  https://msdn.microsoft.com/en-us/library/hh446526.aspx,
A cookie (usually named FedAuth) that can exist either as a persistent or in-memory cookie represents the SharePoint session token. This cookie contains a reference to the SAML token that SharePoint
stores in its token cache. The SAML token contains the claims issued to the user by any external identity and federation providers, and by the internal SharePoint security token service (STS).
It looks like my assumption that FedAuth should change was wrong.
Unfortunately, when I set the cookie lifetime to 20 minutes then start uploading a bunch of files that take more than 20 minutes, the POST requests still get redirected the login page at 20 minutes. That's despite Fiddler showing that a "new" FedAuth
cookie is set.

Answer 1

Have a look here: http://forums.adobe.com/message/2186661#2186661
Ben