A Config Transform to change session state provider to mongo db for a Sitecore8 solution.

session-state-transform.xml

<?xml version="1.0" encoding="utf-8"?>
<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
  <system.web>
    <sessionState mode="Custom" customProvider="mongo" cookieless="false" timeout="20" xdt:Transform="Replace">
      <providers>
        <add name="mongo"
        type="Sitecore.SessionProvider.MongoDB.MongoSessionStateProvider, Sitecore.SessionProvider.MongoDB"
        connectionStringName="session"
        pollingInterval="2"
        compression="true"
        sessionType="private"/>
      </providers>
    </sessionState>
  </system.web>
</configuration>