allowing multiple storage engines in same dbpath creates opportunity for error

Consider a scenario where a customer by mistake starts mongod with the wrong storage engine parameter in a production system. Under the current design mongod will start up and create an empty db in dbpath using that wrong storage engine. Queries to that mongod will begin to fail; it may also begin accepting data and putting it in the wrong db, creating a difficult situation to recover from. We could eliminate this possible source of error by refusing to start if the files currently in dbpath are for a storage engine different from the startup option.