Fix the "on_missing_columns" policy for MongoDB directories

In MongoDB directories implementation, the "on_missing_columns" policy checks if all the entries have all the fields defined in the corresponding schemas.

However, all the fields are not required and therefore the entries in MongoDB could miss some values. This will cause the table drop and therefore the loss of any data added since the previous restart.

For MongoDB, as a noSQL database, there is no notion of columns, so the "on_missing_columns" policy may behave like the "never" policy