Store sessions in a Redis namespace

This makes less of a mess of the Redis root.

config/initializers/session_store.rb

@@ -2,7 +2,7 @@
 
 Gitlab::Application.config.session_store(
   :redis_store, # Using the cookie_store would enable session replay attacks.
-  servers: Gitlab::Application.config.cache_store[1], # re-use the Redis config from the Rails cache store
+  servers: Gitlab::Application.config.cache_store[1].merge(namespace: 'session:gitlab'), # re-use the Redis config from the Rails cache store
   key: '_gitlab_session',
   secure: Gitlab.config.gitlab.https,
   httponly: true,