REACH-34: Remove MongoDB dependency.

This PR adds ability to disable data saving in MongoDB, all necessary data are saved in memory. This mode is designed to work with DynamoServer only.
@pboyer , oh, I missed that this session is some kind of in-memory cache for created workspaces  and it's only for this type of storage, mongoStorage doesn;'t  require it. So I guess it should be a private member of this storage class.