Add option to disable touch

[mmilleruva]

We were having significant scaling problems using express-mysql-session and were considering switching to a different express-session storage adapter. I noticed this adapter https://www.npmjs.com/package/connect-session-sequelize had a disable touch parameter. We have rolled this out in production and it reduced our session DB load by 75%. Obviously every case is unique, we have very long sessions and the session is modified and update quite frequently already so this setting made sense for us.