Session: add ServiceRequest to SessionStore methods

[tp971]

I think it would be useful to add a reference to the ServiceRequest to the SessionStore methods. I'm currently writing a web application with an SQL based session storage backend. My situation is this:

1. I'd like to write certain information about the request (e.g. the remote IP address and the user agent) into the database.
2. I need the ID (primary key) of the session entity (which is an u64) in the rest of the application.
3. I want to only create and update the session if necessary.

This is currently not possible without also being very hacky: because of requirement 3, I only create a session once the user logs in, therefore I cannot just write the information I need into the session everytime because it will trigger a save or update everytime. What I currently do is: in load, I write the session id and token into the returned HashMap, and in save and update, I remove those keys before writing to the database. My authentication middleware then extracts the id from the session and updates the session in the database, which means there are two updates to the same row (in two different transactions). There are also some further writes into the database related to the session, which could theoretically all be put into one transaction without these workarounds. All of this complexity could be eliminated if I had the ServiceRequest in the SessionStore methods.