[data-model] Implement support of custom primary key

[thekangaroofactory]

By default, id is the primary key create along with a new data model.
In case of import, the imported data may already contain a primary key that is not named id

Add a column in the data model to set the primary key (a single value in the column)
After data model is loaded, store the primary key name in cache to avoid duplicating dm[dm$primary, ]$name
Make sure id is not used anywhere in the code; use the primary key name instead