Add a session handler to migrate from pdo to redis, add managed gc times

[BentiGorlich]

• symfony uses the php default garbage collector probabilities, but that is set to 0 in debian and ubuntu, so manage it ourselves so the sessions get cleared out
• add a service definition for redis, which the redis session handler needs, this includes splitting up the dsn in our .env file
• add a MigratingSessionHandler that writes all sessions to the RedisSessionHandler as well, so we can move towards that

[BentiGorlich]

As a note, we moved to the PdoSessionHandler in #1145. Before that we used our redis dsn as the handler_id. I have no idea why that worked at all, though I will be testing the redis sessions out starting tomorrow when all sessions have been duplicated in redis (that is what the MigratingSessionHandler does)