option to only save when `isNew` if populated

[jonathanong]

ie the user would have to set something. i think this is pretty opinionated and should be an option. or do you think it should be default? i guess the only time you wouldn't want this is if you're doing something with the .sids without touching the cookie.

[tj]

maybe .save() wasn't so bad haha, I worry that we're getting to cute just making the whole thing confusing but we'll see!

[jonathanong]

yeah, the way i imagined at first was (for a db session) to be all manual via context methods:

app.use(function* (next) {
  var session = yield this.session()
  this.user = yield User.get(session.userid)
  yield next
  yield this.session.save()
})

but this way is easier. with this method, we won't have an opinions on how sessions are saved, but it'll have a higher learning curve.