[9.x] Add Illuminate\Session\Store::setHandler()

[stancl]

This adds a simple setter for the handler used by Illuminate\Session\Store. The PR doesn't introduce any breaking changes.

The use case is that when you're changing database connections on the fly, the database session driver can run into issues where it tries to run queries on a DB connection that no longer exists.

The session-related middleware instantiates Illuminate\Session\Store for the database driver which uses DatabaseSessionHandler as the $handler. That class stores the connection and is used at the end of the request to write the session data. At that point, the connection that was injected initially may not exist anymore.

By adding the setter, I can reconstruct the DatabaseSessionHandler on the fly and make it use the correct DB connection: https://gist.github.com/stancl/eb6e210b553bf697c13f19ddaf80f539#file-sessiontenancybootstrapper-php-L18-L38

The addition will let me fix archtechx/tenancy#547.

[stancl]

Thanks :)