Disable checking if sessions table exists

[laszlo-dev-13]

Is there a way to disable always checking if the table exists? The reason for this is that we plan to use express-session with dynamoDb and serverless (using aws lambda), so checking if the table exists each time a lambda function gets fired up seems redundant.

[lancegliser]

I'm actually doing the same thing on AWS. It's a cost to run the table check, to be sure. A simple table describe is fairly cheap though.

In the future, if this feature is produced we'll have to adjust for #76 . I expect we could take in initialized?: boolean in the options post refactor.