Add resave option

[interpretor]

Forces the session to be saved back to the session store, even if the session was never modified during the request. This is the current default behavior, with the resave option it is possible to store only changes to the session.

This is also an option of express-session to reduce the risk of race conditions where a client makes two parallel requests and changes made to the session in one request may get overwritten when the other request ends, even if it made no changes.

[BracketJohn]

Thanks @interpretor, another great addition (: We will review the PR shortly and hopefully be able to land the feature then.