Persistent sessions

**Is your feature request related to a problem? Please describe.**
It would be very helpfull if there are persistent sessions in drogon, so for example you dont need to re-login everytime the app restarts. It also would help with load-balancing application so sessions can get syncronized across app instances via the help of a database for example.

**Describe the solution you'd like**
Ideally, there would be a system that let's a app react on each change of a session: create, update (insert & erase), destroy.

I think the best solution would be to implement #1853 in addition with being able to access the session object itself in `AdviceStartSessionCallback` and `AdviceDestroySessionCallback`, i.e. by adding a parameter to pass `SessionPtr`.

If #1853 is not implemented, all of `Session`'s mutation methods would to be changed to allow to notify any custom logic about a changed session, this change notification then maybe could be reacted to with a `drogon::app().registerSessionChangeAdvice()` that also gets the `SessionPtr` as a parameter.

**Describe alternatives you've considered**
Only alternative right now, would to be write an custom solution for this and thus ignoring drogon's session feature completly.