Prepare SessionInfo module to be used in practice

[rphmeier]

After #2092 all runtimes will be aware of Assignment keys.

We have two issues right now:

1. No way exists currently for the AssignmentIds to make their way into the SessionChangeNotifications issued by the Initializer. We'll need to find a way to wire both keys into the initializer, which Substrate's APIs haven't quite been designed for. Then it should be trivial to pull the AssignmentIds out of the SessionChangeNotification in SessionInfo.
2. The SessionInfo module will initially have EarliestSession be 0, but when introduced on a live chain that's been around for several thousand sessions (Kusama, Polkadot), it'll end up in a potentially heavy loop trying to prune all session info from 0..some_high_session_number. We should work around this so enabling the SessionInfo module on existing chains is light.

[ordian]

For 1, an alternative would be to have a runtime module storing those keys for the current session and session info module fetching those keys as we do with the authority discovery keys. This works because of 1 block delay in the session change notification.

[rphmeier]

We could even have the Initializer be that module. We'd just have to implement the OneSessionHandler for some wrapper around Initializer like AssignmentKeyHandler<Initializer>