Use Redis session instance

Based on discussion and the work done in the PoC,

We'll have an ENV variable that needs to be set to change the store that gets loaded in the session_store.rb initializer. Setting the ENV When the ENV variable is set, the store (Gitlab::Redis::Sessions) that's being used is our own implementation MultiStore, but the MultiStore will actually talk directly to SharedState by default. We can enable read-one-fallback and write-both with a feature flag.

We will also replace other usages of Gitlab::Redis::SharedState in ActiveSession and AnonymousSession and all other places where we use sessoins with the Gitlab::Redis::Sessions.